[PATCH 0/3] virtio support cache indirect desc

If the VIRTIO_RING_F_INDIRECT_DESC negotiation succeeds, and the number
of sgs used for sending packets is greater than 1. We must constantly
call __kmalloc/kfree to allocate/release desc.

In the case of extremely fast package delivery, the overhead cannot be
ignored:

  27.46%  [kernel]  [k] virtqueue_add
  16.66%  [kernel]  [k] detach_buf_split
  16.51%  [kernel]  [k] virtnet_xsk_xmit
  14.04%  [kernel]  [k] virtqueue_add_outbuf
   5.18%  [kernel]  [k] __kmalloc
   4.08%  [kernel]  [k] kfree
   2.80%  [kernel]  [k] virtqueue_get_buf_ctx
   2.22%  [kernel]  [k] xsk_tx_peek_desc
   2.08%  [kernel]  [k] memset_erms
   0.83%  [kernel]  [k] virtqueue_kick_prepare
   0.76%  [kernel]  [k] virtnet_xsk_run
   0.62%  [kernel]  [k] __free_old_xmit_ptr
   0.60%  [kernel]  [k] vring_map_one_sg
   0.53%  [kernel]  [k] native_apic_mem_write
   0.46%  [kernel]  [k] sg_next
   0.43%  [kernel]  [k] sg_init_table
   0.41%  [kernel]  [k] kmalloc_slab

This patch adds a cache function to virtio to cache these allocated indirect
desc instead of constantly allocating and releasing desc.


Xuan Zhuo (3):
  virtio: cache indirect desc for split
  virtio: cache indirect desc for packed
  virtio-net: enable virtio indirect cache

 drivers/net/virtio_net.c     |   4 ++
 drivers/virtio/virtio.c      |   6 ++
 drivers/virtio/virtio_ring.c | 120 ++++++++++++++++++++++++++++++-----
 include/linux/virtio.h       |  10 +++
 4 files changed, 123 insertions(+), 17 deletions(-)

--
2.31.0

In the case of using indirect, indirect desc must be allocated and
released each time, which increases a lot of cpu overhead.

Here, a cache is added for indirect. If the number of indirect desc to be
applied for is less than VIRT_QUEUE_CACHE_DESC_NUM, the desc array with
the size of VIRT_QUEUE_CACHE_DESC_NUM is fixed and cached for reuse.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
 drivers/virtio/virtio_ring.c | 57 +++++++++++++++++++++++++++++++-----
 1 file changed, 49 insertions(+), 8 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 0b9a8544b0e8..4fd7bd5bcd70 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c

@@ -1074,10 +1074,45 @@ static void vring_unmap_desc_packed(const struct vring_virtqueue *vq,
 	}
 }
 
-static struct vring_packed_desc *alloc_indirect_packed(unsigned int total_sg,
+static void desc_cache_chain_free_packed(void *chain)
+{
+	struct vring_packed_desc *desc;
+
+	while (chain) {
+		desc = chain;
+		chain = (void *)desc->addr;
+		kfree(desc);
+	}
+}
+
+static void desc_cache_put_packed(struct vring_virtqueue *vq,
+				  struct vring_packed_desc *desc, int n)
+{
+	if (vq->use_desc_cache && n <= VIRT_QUEUE_CACHE_DESC_NUM) {
+		desc->addr = (u64)vq->desc_cache_chain;
+		vq->desc_cache_chain = desc;
+	} else {
+		kfree(desc);
+	}
+}
+
+static struct vring_packed_desc *alloc_indirect_packed(struct vring_virtqueue *vq,
+						       unsigned int total_sg,
 						       gfp_t gfp)
 {
 	struct vring_packed_desc *desc;
+	unsigned int n;
+
+	if (vq->use_desc_cache && total_sg <= VIRT_QUEUE_CACHE_DESC_NUM) {
+		if (vq->desc_cache_chain) {
+			desc = vq->desc_cache_chain;
+			vq->desc_cache_chain = (void *)desc->addr;
+			return desc;
+		}
+		n = VIRT_QUEUE_CACHE_DESC_NUM;
+	} else {
+		n = total_sg;
+	}
 
 	/*
 	 * We require lowmem mappings for the descriptors because

@@ -1086,7 +1121,7 @@ static struct vring_packed_desc *alloc_indirect_packed(unsigned int total_sg,
 	 */
 	gfp &= ~__GFP_HIGHMEM;
 
-	desc = kmalloc_array(total_sg, sizeof(struct vring_packed_desc), gfp);
+	desc = kmalloc_array(n, sizeof(struct vring_packed_desc), gfp);
 
 	return desc;
 }

@@ -1106,7 +1141,7 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
 	dma_addr_t addr;
 
 	head = vq->packed.next_avail_idx;
-	desc = alloc_indirect_packed(total_sg, gfp);
+	desc = alloc_indirect_packed(vq, total_sg, gfp);
 
 	if (unlikely(vq->vq.num_free < 1)) {
 		pr_debug("Can't add buf len 1 - avail = 0\n");

@@ -1197,7 +1232,7 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
 	for (i = 0; i < err_idx; i++)
 		vring_unmap_desc_packed(vq, &desc[i]);
 
-	kfree(desc);
+	desc_cache_put_packed(vq, desc, total_sg);
 
 	END_USE(vq);
 	return -ENOMEM;

@@ -1422,20 +1457,22 @@ static void detach_buf_packed(struct vring_virtqueue *vq,
 	}
 
 	if (vq->indirect) {
-		u32 len;
+		u32 len, n;
 
 		/* Free the indirect table, if any, now that it's unmapped. */
 		desc = state->indir_desc;
 		if (!desc)
 			return;
 
+		n = len / sizeof(struct vring_packed_desc);
+
 		if (vq->use_dma_api) {
 			len = vq->packed.desc_extra[id].len;
-			for (i = 0; i < len / sizeof(struct vring_packed_desc);
-					i++)
+			for (i = 0; i < n; i++)
 				vring_unmap_desc_packed(vq, &desc[i]);
 		}
-		kfree(desc);
+
+		desc_cache_put_packed(vq, desc, n);
 		state->indir_desc = NULL;
 	} else if (ctx) {
 		*ctx = state->indir_desc;

@@ -1753,6 +1790,8 @@ static struct virtqueue *vring_create_virtqueue_packed(
 	vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC) &&
 		!context;
 	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
+	vq->desc_cache_chain = NULL;
+	vq->use_desc_cache = vdev->desc_cache;
 
 	if (virtio_has_feature(vdev, VIRTIO_F_ORDER_PLATFORM))
 		vq->weak_barriers = false;

@@ -2374,6 +2413,8 @@ void vring_del_virtqueue(struct virtqueue *_vq)
 		kfree(vq->split.desc_state);
 		kfree(vq->split.desc_extra);
 		desc_cache_chain_free_split(vq->desc_cache_chain);
+	} else {
+		desc_cache_chain_free_packed(vq->desc_cache_chain);
 	}
 	kfree(vq);
 }

-- 
2.31.0

In the case of using indirect, indirect desc must be allocated and
released each time, which increases a lot of cpu overhead.

Here, a cache is added for indirect. If the number of indirect desc to be
applied for is less than VIRT_QUEUE_CACHE_DESC_NUM, the desc array with
the size of VIRT_QUEUE_CACHE_DESC_NUM is fixed and cached for reuse.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
 drivers/virtio/virtio.c      |  6 ++++
 drivers/virtio/virtio_ring.c | 63 ++++++++++++++++++++++++++++++------
 include/linux/virtio.h       | 10 ++++++
 3 files changed, 70 insertions(+), 9 deletions(-)

diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index 0a5b54034d4b..04bcb74e5b9a 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c

@@ -431,6 +431,12 @@ bool is_virtio_device(struct device *dev)
 }
 EXPORT_SYMBOL_GPL(is_virtio_device);
 
+void virtio_use_desc_cache(struct virtio_device *dev, bool val)
+{
+	dev->desc_cache = val;
+}
+EXPORT_SYMBOL_GPL(virtio_use_desc_cache);
+
 void unregister_virtio_device(struct virtio_device *dev)
 {
 	int index = dev->index; /* save for after device release */

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index dd95dfd85e98..0b9a8544b0e8 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c

@@ -117,6 +117,10 @@ struct vring_virtqueue {
 	/* Hint for event idx: already triggered no need to disable. */
 	bool event_triggered;
 
+	/* Is indirect cache used? */
+	bool use_desc_cache;
+	void *desc_cache_chain;
+
 	union {
 		/* Available for split ring */
 		struct {

@@ -423,12 +427,47 @@ static unsigned int vring_unmap_one_split(const struct vring_virtqueue *vq,
 	return extra[i].next;
 }
 
-static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
+#define VIRT_QUEUE_CACHE_DESC_NUM 4
+
+static void desc_cache_chain_free_split(void *chain)
+{
+	struct vring_desc *desc;
+
+	while (chain) {
+		desc = chain;
+		chain = (void *)desc->addr;
+		kfree(desc);
+	}
+}
+
+static void desc_cache_put_split(struct vring_virtqueue *vq,
+				 struct vring_desc *desc, int n)
+{
+	if (vq->use_desc_cache && n <= VIRT_QUEUE_CACHE_DESC_NUM) {
+		desc->addr = (u64)vq->desc_cache_chain;
+		vq->desc_cache_chain = desc;
+	} else {
+		kfree(desc);
+	}
+}
+
+static struct vring_desc *alloc_indirect_split(struct vring_virtqueue *vq,
 					       unsigned int total_sg,
 					       gfp_t gfp)
 {
 	struct vring_desc *desc;
-	unsigned int i;
+	unsigned int i, n;
+
+	if (vq->use_desc_cache && total_sg <= VIRT_QUEUE_CACHE_DESC_NUM) {
+		if (vq->desc_cache_chain) {
+			desc = vq->desc_cache_chain;
+			vq->desc_cache_chain = (void *)desc->addr;
+			goto got;
+		}
+		n = VIRT_QUEUE_CACHE_DESC_NUM;
+	} else {
+		n = total_sg;
+	}
 
 	/*
 	 * We require lowmem mappings for the descriptors because

@@ -437,12 +476,13 @@ static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
 	 */
 	gfp &= ~__GFP_HIGHMEM;
 
-	desc = kmalloc_array(total_sg, sizeof(struct vring_desc), gfp);
+	desc = kmalloc_array(n, sizeof(struct vring_desc), gfp);
 	if (!desc)
 		return NULL;
 
+got:
 	for (i = 0; i < total_sg; i++)
-		desc[i].next = cpu_to_virtio16(_vq->vdev, i + 1);
+		desc[i].next = cpu_to_virtio16(vq->vq.vdev, i + 1);
 	return desc;
 }
 

@@ -508,7 +548,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
 	head = vq->free_head;
 
 	if (virtqueue_use_indirect(_vq, total_sg))
-		desc = alloc_indirect_split(_vq, total_sg, gfp);
+		desc = alloc_indirect_split(vq, total_sg, gfp);
 	else {
 		desc = NULL;
 		WARN_ON_ONCE(total_sg > vq->split.vring.num && !vq->indirect);

@@ -652,7 +692,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
 	}
 
 	if (indirect)
-		kfree(desc);
+		desc_cache_put_split(vq, desc, total_sg);
 
 	END_USE(vq);
 	return -ENOMEM;

@@ -717,7 +757,7 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
 	if (vq->indirect) {
 		struct vring_desc *indir_desc =
 				vq->split.desc_state[head].indir_desc;
-		u32 len;
+		u32 len, n;
 
 		/* Free the indirect table, if any, now that it's unmapped. */
 		if (!indir_desc)

@@ -729,10 +769,12 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
 				VRING_DESC_F_INDIRECT));
 		BUG_ON(len == 0 || len % sizeof(struct vring_desc));
 
-		for (j = 0; j < len / sizeof(struct vring_desc); j++)
+		n = len / sizeof(struct vring_desc);
+
+		for (j = 0; j < n; j++)
 			vring_unmap_one_split_indirect(vq, &indir_desc[j]);
 
-		kfree(indir_desc);
+		desc_cache_put_split(vq, indir_desc, n);
 		vq->split.desc_state[head].indir_desc = NULL;
 	} else if (ctx) {
 		*ctx = vq->split.desc_state[head].indir_desc;

@@ -2199,6 +2241,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
 	vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC) &&
 		!context;
 	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
+	vq->desc_cache_chain = NULL;
+	vq->use_desc_cache = vdev->desc_cache;
 
 	if (virtio_has_feature(vdev, VIRTIO_F_ORDER_PLATFORM))
 		vq->weak_barriers = false;

@@ -2329,6 +2373,7 @@ void vring_del_virtqueue(struct virtqueue *_vq)
 	if (!vq->packed_ring) {
 		kfree(vq->split.desc_state);
 		kfree(vq->split.desc_extra);
+		desc_cache_chain_free_split(vq->desc_cache_chain);
 	}
 	kfree(vq);
 }

diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index 41edbc01ffa4..d84b7b8f4070 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h

@@ -109,6 +109,7 @@ struct virtio_device {
 	bool failed;
 	bool config_enabled;
 	bool config_change_pending;
+	bool desc_cache;
 	spinlock_t config_lock;
 	spinlock_t vqs_list_lock; /* Protects VQs list access */
 	struct device dev;

@@ -130,6 +131,15 @@ int register_virtio_device(struct virtio_device *dev);
 void unregister_virtio_device(struct virtio_device *dev);
 bool is_virtio_device(struct device *dev);
 
+/**
+ * virtio_use_desc_cache - virtio ring use desc cache
+ *
+ * virtio will cache the allocated indirect desc.
+ *
+ * This function must be called before find_vqs.
+ */
+void virtio_use_desc_cache(struct virtio_device *dev, bool val);
+
 void virtio_break_device(struct virtio_device *dev);
 
 void virtio_config_changed(struct virtio_device *dev);

-- 
2.31.0

If the VIRTIO_RING_F_INDIRECT_DESC negotiation succeeds, and the number
of sgs used for sending packets is greater than 1. We must constantly
call __kmalloc/kfree to allocate/release desc.

In the case of extremely fast package delivery, the overhead cannot be
ignored:

  27.46%  [kernel]  [k] virtqueue_add
  16.66%  [kernel]  [k] detach_buf_split
  16.51%  [kernel]  [k] virtnet_xsk_xmit
  14.04%  [kernel]  [k] virtqueue_add_outbuf
   5.18%  [kernel]  [k] __kmalloc
   4.08%  [kernel]  [k] kfree
   2.80%  [kernel]  [k] virtqueue_get_buf_ctx
   2.22%  [kernel]  [k] xsk_tx_peek_desc
   2.08%  [kernel]  [k] memset_erms
   0.83%  [kernel]  [k] virtqueue_kick_prepare
   0.76%  [kernel]  [k] virtnet_xsk_run
   0.62%  [kernel]  [k] __free_old_xmit_ptr
   0.60%  [kernel]  [k] vring_map_one_sg
   0.53%  [kernel]  [k] native_apic_mem_write
   0.46%  [kernel]  [k] sg_next
   0.43%  [kernel]  [k] sg_init_table
   0.41%  [kernel]  [k] kmalloc_slab

Compared to not using virtio indirect cache, virtio-net can get a 16%
performance improvement when using indirect desc cache.

In the test case, the CPU where the package is sent has reached 100%.
The following are the PPS in two cases:

    indirect desc cache  | no cache
    3074658              | 2685132
    3111866              | 2666118
    3152527              | 2653632
    3125867              | 2669820
    3027147              | 2644464
    3069211              | 2669777
    3038522              | 2675645
    3034507              | 2671302
    3102257              | 2685504
    3083712              | 2692800
    3051771              | 2676928
    3080684              | 2695040
    3147816              | 2720876
    3123887              | 2705492
    3180963              | 2699520
    3191579              | 2676480
    3161670              | 2686272
    3189768              | 2692588
    3174272              | 2686692
    3143434              | 2682416

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
 drivers/net/virtio_net.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 4ad25a8b0870..0ec29cf90d0a 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -27,9 +27,11 @@ static int napi_weight = NAPI_POLL_WEIGHT;
 module_param(napi_weight, int, 0444);
 
 static bool csum = true, gso = true, napi_tx = true;
+static bool virtio_desc_cache = true;
 module_param(csum, bool, 0444);
 module_param(gso, bool, 0444);
 module_param(napi_tx, bool, 0644);
+module_param(virtio_desc_cache, bool, 0644);
 
 /* FIXME: MTU in config. */
 #define GOOD_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)

@@ -3214,6 +3216,8 @@ static int virtnet_probe(struct virtio_device *vdev)
 		vi->curr_queue_pairs = num_online_cpus();
 	vi->max_queue_pairs = max_queue_pairs;
 
+	virtio_use_desc_cache(vdev, virtio_desc_cache);
+
 	/* Allocate/initialize the rx/tx queues, and invoke find_vqs */
 	err = init_vqs(vi);
 	if (err)

-- 
2.31.0

On Wed, Oct 27, 2021 at 02:19:11PM +0800, Xuan Zhuo wrote:

Let's use llist_head and friends please (I am guessing we want
single-linked to avoid writing into indirect buffer after use,
invalidating the cache, but please document why in a comment).  Do not
open-code it.

Also hide all casts in inline wrappers.

Add an inline wrapper for checking sg versus VIRT_QUEUE_CACHE_DESC_NUM.

+
+static void desc_cache_chain_free_split(void *chain)
+{
+	struct vring_desc *desc;
+
+	while (chain) {
+		desc = chain;
+		chain = (void *)desc->addr;
+		kfree(desc);
+	}
+}
+
+static void desc_cache_put_split(struct vring_virtqueue *vq,
+				 struct vring_desc *desc, int n)
+{
+	if (vq->use_desc_cache && n <= VIRT_QUEUE_CACHE_DESC_NUM) {
+		desc->addr = (u64)vq->desc_cache_chain;
+		vq->desc_cache_chain = desc;
+	} else {
+		kfree(desc);
+	}
+}

+
+static struct vring_desc *alloc_indirect_split(struct vring_virtqueue *vq,
 					       unsigned int total_sg,
 					       gfp_t gfp)
 {
 	struct vring_desc *desc;
-	unsigned int i;
+	unsigned int i, n;
+
+	if (vq->use_desc_cache && total_sg <= VIRT_QUEUE_CACHE_DESC_NUM) {
+		if (vq->desc_cache_chain) {
+			desc = vq->desc_cache_chain;
+			vq->desc_cache_chain = (void *)desc->addr;
+			goto got;
+		}
+		n = VIRT_QUEUE_CACHE_DESC_NUM;

Hmm. This will allocate more entries than actually used. Why do it?

On Wed, 27 Oct 2021 14:19:13 +0800 Xuan Zhuo wrote:

+static bool virtio_desc_cache = true;
 module_param(csum, bool, 0444);
 module_param(gso, bool, 0444);
 module_param(napi_tx, bool, 0644);
+module_param(virtio_desc_cache, bool, 0644);

Can this be an ethtool priv flag? module params are discouraged because
they can't be controlled per-netdev.

On 10/26/21 11:19 PM, Xuan Zhuo wrote:

How about to make the VIRT_QUEUE_CACHE_DESC_NUM configurable (at least during
driver probing) unless there is a reason that the default value is 4.

Thank you very much!

Dongli Zhang

On Wed, Oct 27, 2021 at 02:19:11PM +0800, Xuan Zhuo wrote:

So I have a question here. What happens if we just do:

if (n <= VIRT_QUEUE_CACHE_DESC_NUM) {
	return kmem_cache_alloc(VIRT_QUEUE_CACHE_DESC_NUM * sizeof desc, gfp)
} else {
	return kmalloc_arrat(n, sizeof desc, gfp)
}

A small change and won't we reap most performance benefits?

-- 
MST

On Wed, Oct 27, 2021 at 09:33:46AM -0700, Dongli Zhang wrote:

On 10/26/21 11:19 PM, Xuan Zhuo wrote:

quoted

In the case of using indirect, indirect desc must be allocated and
released each time, which increases a lot of cpu overhead.

Here, a cache is added for indirect. If the number of indirect desc to be
applied for is less than VIRT_QUEUE_CACHE_DESC_NUM, the desc array with
the size of VIRT_QUEUE_CACHE_DESC_NUM is fixed and cached for reuse.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
 drivers/virtio/virtio.c      |  6 ++++
 drivers/virtio/virtio_ring.c | 63 ++++++++++++++++++++++++++++++------
 include/linux/virtio.h       | 10 ++++++
 3 files changed, 70 insertions(+), 9 deletions(-)

diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index 0a5b54034d4b..04bcb74e5b9a 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c

@@ -431,6 +431,12 @@ bool is_virtio_device(struct device *dev)
 }
 EXPORT_SYMBOL_GPL(is_virtio_device);
 
+void virtio_use_desc_cache(struct virtio_device *dev, bool val)
+{
+	dev->desc_cache = val;
+}
+EXPORT_SYMBOL_GPL(virtio_use_desc_cache);
+
 void unregister_virtio_device(struct virtio_device *dev)
 {
 	int index = dev->index; /* save for after device release */

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index dd95dfd85e98..0b9a8544b0e8 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c

@@ -117,6 +117,10 @@ struct vring_virtqueue {
 	/* Hint for event idx: already triggered no need to disable. */
 	bool event_triggered;
 
+	/* Is indirect cache used? */
+	bool use_desc_cache;
+	void *desc_cache_chain;
+
 	union {
 		/* Available for split ring */
 		struct {

@@ -423,12 +427,47 @@ static unsigned int vring_unmap_one_split(const struct vring_virtqueue *vq,
 	return extra[i].next;
 }
 
-static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
+#define VIRT_QUEUE_CACHE_DESC_NUM 4
+
+static void desc_cache_chain_free_split(void *chain)
+{
+	struct vring_desc *desc;
+
+	while (chain) {
+		desc = chain;
+		chain = (void *)desc->addr;
+		kfree(desc);
+	}
+}
+
+static void desc_cache_put_split(struct vring_virtqueue *vq,
+				 struct vring_desc *desc, int n)
+{
+	if (vq->use_desc_cache && n <= VIRT_QUEUE_CACHE_DESC_NUM) {
+		desc->addr = (u64)vq->desc_cache_chain;
+		vq->desc_cache_chain = desc;
+	} else {
+		kfree(desc);
+	}
+}
+
+static struct vring_desc *alloc_indirect_split(struct vring_virtqueue *vq,
 					       unsigned int total_sg,
 					       gfp_t gfp)
 {
 	struct vring_desc *desc;
-	unsigned int i;
+	unsigned int i, n;
+
+	if (vq->use_desc_cache && total_sg <= VIRT_QUEUE_CACHE_DESC_NUM) {
+		if (vq->desc_cache_chain) {
+			desc = vq->desc_cache_chain;
+			vq->desc_cache_chain = (void *)desc->addr;
+			goto got;
+		}
+		n = VIRT_QUEUE_CACHE_DESC_NUM;

How about to make the VIRT_QUEUE_CACHE_DESC_NUM configurable (at least during
driver probing) unless there is a reason that the default value is 4.

Thank you very much!

Dongli Zhang

I would start with some experimentation showing that it actually makes a
difference in performance.

quoted

+	} else {
+		n = total_sg;
+	}
 
 	/*
 	 * We require lowmem mappings for the descriptors because

@@ -437,12 +476,13 @@ static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
 	 */
 	gfp &= ~__GFP_HIGHMEM;
 
-	desc = kmalloc_array(total_sg, sizeof(struct vring_desc), gfp);
+	desc = kmalloc_array(n, sizeof(struct vring_desc), gfp);
 	if (!desc)
 		return NULL;
 
+got:
 	for (i = 0; i < total_sg; i++)
-		desc[i].next = cpu_to_virtio16(_vq->vdev, i + 1);
+		desc[i].next = cpu_to_virtio16(vq->vq.vdev, i + 1);
 	return desc;
 }
 

@@ -508,7 +548,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
 	head = vq->free_head;
 
 	if (virtqueue_use_indirect(_vq, total_sg))
-		desc = alloc_indirect_split(_vq, total_sg, gfp);
+		desc = alloc_indirect_split(vq, total_sg, gfp);
 	else {
 		desc = NULL;
 		WARN_ON_ONCE(total_sg > vq->split.vring.num && !vq->indirect);

@@ -652,7 +692,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
 	}
 
 	if (indirect)
-		kfree(desc);
+		desc_cache_put_split(vq, desc, total_sg);
 
 	END_USE(vq);
 	return -ENOMEM;

@@ -717,7 +757,7 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
 	if (vq->indirect) {
 		struct vring_desc *indir_desc =
 				vq->split.desc_state[head].indir_desc;
-		u32 len;
+		u32 len, n;
 
 		/* Free the indirect table, if any, now that it's unmapped. */
 		if (!indir_desc)

@@ -729,10 +769,12 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
 				VRING_DESC_F_INDIRECT));
 		BUG_ON(len == 0 || len % sizeof(struct vring_desc));
 
-		for (j = 0; j < len / sizeof(struct vring_desc); j++)
+		n = len / sizeof(struct vring_desc);
+
+		for (j = 0; j < n; j++)
 			vring_unmap_one_split_indirect(vq, &indir_desc[j]);
 
-		kfree(indir_desc);
+		desc_cache_put_split(vq, indir_desc, n);
 		vq->split.desc_state[head].indir_desc = NULL;
 	} else if (ctx) {
 		*ctx = vq->split.desc_state[head].indir_desc;

@@ -2199,6 +2241,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
 	vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC) &&
 		!context;
 	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
+	vq->desc_cache_chain = NULL;
+	vq->use_desc_cache = vdev->desc_cache;
 
 	if (virtio_has_feature(vdev, VIRTIO_F_ORDER_PLATFORM))
 		vq->weak_barriers = false;

@@ -2329,6 +2373,7 @@ void vring_del_virtqueue(struct virtqueue *_vq)
 	if (!vq->packed_ring) {
 		kfree(vq->split.desc_state);
 		kfree(vq->split.desc_extra);
+		desc_cache_chain_free_split(vq->desc_cache_chain);
 	}
 	kfree(vq);
 }

diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index 41edbc01ffa4..d84b7b8f4070 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h

@@ -109,6 +109,7 @@ struct virtio_device {
 	bool failed;
 	bool config_enabled;
 	bool config_change_pending;
+	bool desc_cache;
 	spinlock_t config_lock;
 	spinlock_t vqs_list_lock; /* Protects VQs list access */
 	struct device dev;

@@ -130,6 +131,15 @@ int register_virtio_device(struct virtio_device *dev);
 void unregister_virtio_device(struct virtio_device *dev);
 bool is_virtio_device(struct device *dev);
 
+/**
+ * virtio_use_desc_cache - virtio ring use desc cache
+ *
+ * virtio will cache the allocated indirect desc.
+ *
+ * This function must be called before find_vqs.
+ */
+void virtio_use_desc_cache(struct virtio_device *dev, bool val);
+
 void virtio_break_device(struct virtio_device *dev);
 
 void virtio_config_changed(struct virtio_device *dev);

Hi Xuan,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on horms-ipvs/master]
[also build test WARNING on linus/master v5.15-rc7 next-20211027]
[cannot apply to mst-vhost/linux-next]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Xuan-Zhuo/virtio-support-cache-indirect-desc/20211027-142025
base:   https://git.kernel.org/pub/scm/linux/kernel/git/horms/ipvs.git master
config: arm-defconfig (attached as .config)
compiler: arm-linux-gnueabi-gcc (GCC) 11.2.0
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # https://github.com/0day-ci/linux/commit/8935e116c155fb7d484bad35a42b2ca98f75e384
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Xuan-Zhuo/virtio-support-cache-indirect-desc/20211027-142025
        git checkout 8935e116c155fb7d484bad35a42b2ca98f75e384
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-11.2.0 make.cross ARCH=arm 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <redacted>

All warnings (new ones prefixed by >>):

   drivers/virtio/virtio_ring.c: In function 'desc_cache_chain_free_split':

quoted

drivers/virtio/virtio_ring.c:438:25: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

     438 |                 chain = (void *)desc->addr;
         |                         ^
   drivers/virtio/virtio_ring.c: In function 'desc_cache_put_split':

quoted

drivers/virtio/virtio_ring.c:447:30: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]

     447 |                 desc->addr = (u64)vq->desc_cache_chain;
         |                              ^
   drivers/virtio/virtio_ring.c: In function 'alloc_indirect_split':
   drivers/virtio/virtio_ring.c:464:48: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
     464 |                         vq->desc_cache_chain = (void *)desc->addr;
         |                                                ^


vim +438 drivers/virtio/virtio_ring.c

   431	
   432	static void desc_cache_chain_free_split(void *chain)
   433	{
   434		struct vring_desc *desc;
   435	
   436		while (chain) {
   437			desc = chain;
 > 438			chain = (void *)desc->addr;
   439			kfree(desc);
   440		}
   441	}
   442	
   443	static void desc_cache_put_split(struct vring_virtqueue *vq,
   444					 struct vring_desc *desc, int n)
   445	{
   446		if (vq->use_desc_cache && n <= VIRT_QUEUE_CACHE_DESC_NUM) {
 > 447			desc->addr = (u64)vq->desc_cache_chain;
   448			vq->desc_cache_chain = desc;
   449		} else {
   450			kfree(desc);
   451		}
   452	}
   453	

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org

Hi Xuan,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on horms-ipvs/master]
[also build test WARNING on linus/master v5.15-rc7]
[cannot apply to mst-vhost/linux-next next-20211027]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Xuan-Zhuo/virtio-support-cache-indirect-desc/20211027-142025
base:   https://git.kernel.org/pub/scm/linux/kernel/git/horms/ipvs.git master
config: hexagon-randconfig-r045-20211027 (attached as .config)
compiler: clang version 14.0.0 (https://github.com/llvm/llvm-project 5db7568a6a1fcb408eb8988abdaff2a225a8eb72)
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # https://github.com/0day-ci/linux/commit/bb65ceda850ed4592d8a940e01926d5e3d33ae92
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Xuan-Zhuo/virtio-support-cache-indirect-desc/20211027-142025
        git checkout bb65ceda850ed4592d8a940e01926d5e3d33ae92
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 ARCH=hexagon 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <redacted>

All warnings (new ones prefixed by >>):

quoted

drivers/virtio/virtio_ring.c:1467:7: warning: variable 'len' is uninitialized when used here [-Wuninitialized]

                   n = len / sizeof(struct vring_packed_desc);
                       ^~~
   drivers/virtio/virtio_ring.c:1460:10: note: initialize the variable 'len' to silence this warning
                   u32 len, n;
                          ^
                           = 0
   1 warning generated.


vim +/len +1467 drivers/virtio/virtio_ring.c

  1433	
  1434	static void detach_buf_packed(struct vring_virtqueue *vq,
  1435				      unsigned int id, void **ctx)
  1436	{
  1437		struct vring_desc_state_packed *state = NULL;
  1438		struct vring_packed_desc *desc;
  1439		unsigned int i, curr;
  1440	
  1441		state = &vq->packed.desc_state[id];
  1442	
  1443		/* Clear data ptr. */
  1444		state->data = NULL;
  1445	
  1446		vq->packed.desc_extra[state->last].next = vq->free_head;
  1447		vq->free_head = id;
  1448		vq->vq.num_free += state->num;
  1449	
  1450		if (unlikely(vq->use_dma_api)) {
  1451			curr = id;
  1452			for (i = 0; i < state->num; i++) {
  1453				vring_unmap_state_packed(vq,
  1454					&vq->packed.desc_extra[curr]);
  1455				curr = vq->packed.desc_extra[curr].next;
  1456			}
  1457		}
  1458	
  1459		if (vq->indirect) {
  1460			u32 len, n;
  1461	
  1462			/* Free the indirect table, if any, now that it's unmapped. */
  1463			desc = state->indir_desc;
  1464			if (!desc)
  1465				return;
  1466	

1467			n = len / sizeof(struct vring_packed_desc);

  1468	
  1469			if (vq->use_dma_api) {
  1470				len = vq->packed.desc_extra[id].len;
  1471				for (i = 0; i < n; i++)
  1472					vring_unmap_desc_packed(vq, &desc[i]);
  1473			}
  1474	
  1475			desc_cache_put_packed(vq, desc, n);
  1476			state->indir_desc = NULL;
  1477		} else if (ctx) {
  1478			*ctx = state->indir_desc;
  1479		}
  1480	}
  1481	

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org

Hi Xuan,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on horms-ipvs/master]
[also build test WARNING on linus/master v5.15-rc7 next-20211027]
[cannot apply to mst-vhost/linux-next]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Xuan-Zhuo/virtio-support-cache-indirect-desc/20211027-142025
base:   https://git.kernel.org/pub/scm/linux/kernel/git/horms/ipvs.git master
config: i386-randconfig-s002-20211027 (attached as .config)
compiler: gcc-9 (Debian 9.3.0-22) 9.3.0
reproduce:
        # apt-get install sparse
        # sparse version: v0.6.4-dirty
        # https://github.com/0day-ci/linux/commit/8935e116c155fb7d484bad35a42b2ca98f75e384
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Xuan-Zhuo/virtio-support-cache-indirect-desc/20211027-142025
        git checkout 8935e116c155fb7d484bad35a42b2ca98f75e384
        # save the attached .config to linux build tree
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' O=build_dir ARCH=i386 SHELL=/bin/bash drivers/virtio/

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <redacted>


sparse warnings: (new ones prefixed by >>)

quoted

drivers/virtio/virtio_ring.c:438:26: sparse: sparse: cast from restricted __virtio64
drivers/virtio/virtio_ring.c:447:28: sparse: sparse: incorrect type in assignment (different base types) @@     expected restricted __virtio64 [usertype] addr @@     got unsigned long long [usertype] @@

   drivers/virtio/virtio_ring.c:447:28: sparse:     expected restricted __virtio64 [usertype] addr
   drivers/virtio/virtio_ring.c:447:28: sparse:     got unsigned long long [usertype]
   drivers/virtio/virtio_ring.c:464:49: sparse: sparse: cast from restricted __virtio64

vim +438 drivers/virtio/virtio_ring.c

   431	
   432	static void desc_cache_chain_free_split(void *chain)
   433	{
   434		struct vring_desc *desc;
   435	
   436		while (chain) {
   437			desc = chain;
 > 438			chain = (void *)desc->addr;
   439			kfree(desc);
   440		}
   441	}
   442	
   443	static void desc_cache_put_split(struct vring_virtqueue *vq,
   444					 struct vring_desc *desc, int n)
   445	{
   446		if (vq->use_desc_cache && n <= VIRT_QUEUE_CACHE_DESC_NUM) {
 > 447			desc->addr = (u64)vq->desc_cache_chain;
   448			vq->desc_cache_chain = desc;
   449		} else {
   450			kfree(desc);
   451		}
   452	}
   453	

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org

On Thu, Oct 28, 2021 at 1:07 AM Michael S. Tsirkin [off-list ref] wrote:

On Wed, Oct 27, 2021 at 02:19:11PM +0800, Xuan Zhuo wrote:

quoted

In the case of using indirect, indirect desc must be allocated and
released each time, which increases a lot of cpu overhead.

Here, a cache is added for indirect. If the number of indirect desc to be
applied for is less than VIRT_QUEUE_CACHE_DESC_NUM, the desc array with
the size of VIRT_QUEUE_CACHE_DESC_NUM is fixed and cached for reuse.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
 drivers/virtio/virtio.c      |  6 ++++
 drivers/virtio/virtio_ring.c | 63 ++++++++++++++++++++++++++++++------
 include/linux/virtio.h       | 10 ++++++
 3 files changed, 70 insertions(+), 9 deletions(-)

diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index 0a5b54034d4b..04bcb74e5b9a 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c

@@ -431,6 +431,12 @@ bool is_virtio_device(struct device *dev)
 }
 EXPORT_SYMBOL_GPL(is_virtio_device);

+void virtio_use_desc_cache(struct virtio_device *dev, bool val)
+{
+     dev->desc_cache = val;
+}
+EXPORT_SYMBOL_GPL(virtio_use_desc_cache);
+
 void unregister_virtio_device(struct virtio_device *dev)
 {
      int index = dev->index; /* save for after device release */

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index dd95dfd85e98..0b9a8544b0e8 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c

@@ -117,6 +117,10 @@ struct vring_virtqueue {
      /* Hint for event idx: already triggered no need to disable. */
      bool event_triggered;

+     /* Is indirect cache used? */
+     bool use_desc_cache;
+     void *desc_cache_chain;
+
      union {
              /* Available for split ring */
              struct {

@@ -423,12 +427,47 @@ static unsigned int vring_unmap_one_split(const struct vring_virtqueue *vq,
      return extra[i].next;
 }

-static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
+#define VIRT_QUEUE_CACHE_DESC_NUM 4
+
+static void desc_cache_chain_free_split(void *chain)
+{
+     struct vring_desc *desc;
+
+     while (chain) {
+             desc = chain;
+             chain = (void *)desc->addr;
+             kfree(desc);
+     }
+}
+
+static void desc_cache_put_split(struct vring_virtqueue *vq,
+                              struct vring_desc *desc, int n)
+{
+     if (vq->use_desc_cache && n <= VIRT_QUEUE_CACHE_DESC_NUM) {
+             desc->addr = (u64)vq->desc_cache_chain;
+             vq->desc_cache_chain = desc;
+     } else {
+             kfree(desc);
+     }
+}
+

So I have a question here. What happens if we just do:

if (n <= VIRT_QUEUE_CACHE_DESC_NUM) {
        return kmem_cache_alloc(VIRT_QUEUE_CACHE_DESC_NUM * sizeof desc, gfp)
} else {
        return kmalloc_arrat(n, sizeof desc, gfp)
}

A small change and won't we reap most performance benefits?

Yes, I think we need a benchmark to use private cache to see how much
it can help.

Thanks

--
MST

Hi Xuan,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on horms-ipvs/master]
[also build test WARNING on linus/master v5.15-rc7]
[cannot apply to mst-vhost/linux-next next-20211027]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Xuan-Zhuo/virtio-support-cache-indirect-desc/20211027-142025
base:   https://git.kernel.org/pub/scm/linux/kernel/git/horms/ipvs.git master
config: i386-randconfig-s002-20211027 (attached as .config)
compiler: gcc-9 (Debian 9.3.0-22) 9.3.0
reproduce:
        # apt-get install sparse
        # sparse version: v0.6.4-dirty
        # https://github.com/0day-ci/linux/commit/bb65ceda850ed4592d8a940e01926d5e3d33ae92
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Xuan-Zhuo/virtio-support-cache-indirect-desc/20211027-142025
        git checkout bb65ceda850ed4592d8a940e01926d5e3d33ae92
        # save the attached .config to linux build tree
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' O=build_dir ARCH=i386 SHELL=/bin/bash drivers/virtio/

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <redacted>


sparse warnings: (new ones prefixed by >>)
   drivers/virtio/virtio_ring.c:438:26: sparse: sparse: cast from restricted __virtio64
   drivers/virtio/virtio_ring.c:447:28: sparse: sparse: incorrect type in assignment (different base types) @@     expected restricted __virtio64 [usertype] addr @@     got unsigned long long [usertype] @@
   drivers/virtio/virtio_ring.c:447:28: sparse:     expected restricted __virtio64 [usertype] addr
   drivers/virtio/virtio_ring.c:447:28: sparse:     got unsigned long long [usertype]
   drivers/virtio/virtio_ring.c:464:49: sparse: sparse: cast from restricted __virtio64

quoted

drivers/virtio/virtio_ring.c:1083:26: sparse: sparse: cast from restricted __le64
drivers/virtio/virtio_ring.c:1092:28: sparse: sparse: incorrect type in assignment (different base types) @@     expected restricted __le64 [usertype] addr @@     got unsigned long long [usertype] @@

   drivers/virtio/virtio_ring.c:1092:28: sparse:     expected restricted __le64 [usertype] addr
   drivers/virtio/virtio_ring.c:1092:28: sparse:     got unsigned long long [usertype]
   drivers/virtio/virtio_ring.c:1109:49: sparse: sparse: cast from restricted __le64

vim +1083 drivers/virtio/virtio_ring.c

  1076	
  1077	static void desc_cache_chain_free_packed(void *chain)
  1078	{
  1079		struct vring_packed_desc *desc;
  1080	
  1081		while (chain) {
  1082			desc = chain;

1083			chain = (void *)desc->addr;

  1084			kfree(desc);
  1085		}
  1086	}
  1087	
  1088	static void desc_cache_put_packed(struct vring_virtqueue *vq,
  1089					  struct vring_packed_desc *desc, int n)
  1090	{
  1091		if (vq->use_desc_cache && n <= VIRT_QUEUE_CACHE_DESC_NUM) {

1092			desc->addr = (u64)vq->desc_cache_chain;

  1093			vq->desc_cache_chain = desc;
  1094		} else {
  1095			kfree(desc);
  1096		}
  1097	}
  1098	

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org

Hi Xuan,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on horms-ipvs/master]
[also build test ERROR on linus/master v5.15-rc7]
[cannot apply to mst-vhost/linux-next next-20211027]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Xuan-Zhuo/virtio-support-cache-indirect-desc/20211027-142025
base:   https://git.kernel.org/pub/scm/linux/kernel/git/horms/ipvs.git master
config: i386-buildonly-randconfig-r005-20211027 (attached as .config)
compiler: clang version 14.0.0 (https://github.com/llvm/llvm-project 5db7568a6a1fcb408eb8988abdaff2a225a8eb72)
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # https://github.com/0day-ci/linux/commit/bb65ceda850ed4592d8a940e01926d5e3d33ae92
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Xuan-Zhuo/virtio-support-cache-indirect-desc/20211027-142025
        git checkout bb65ceda850ed4592d8a940e01926d5e3d33ae92
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 ARCH=i386 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <redacted>

All errors (new ones prefixed by >>):

quoted

drivers/virtio/virtio_ring.c:1467:7: error: variable 'len' is uninitialized when used here [-Werror,-Wuninitialized]

                   n = len / sizeof(struct vring_packed_desc);
                       ^~~
   drivers/virtio/virtio_ring.c:1460:10: note: initialize the variable 'len' to silence this warning
                   u32 len, n;
                          ^
                           = 0
   1 error generated.


vim +/len +1467 drivers/virtio/virtio_ring.c

  1433	
  1434	static void detach_buf_packed(struct vring_virtqueue *vq,
  1435				      unsigned int id, void **ctx)
  1436	{
  1437		struct vring_desc_state_packed *state = NULL;
  1438		struct vring_packed_desc *desc;
  1439		unsigned int i, curr;
  1440	
  1441		state = &vq->packed.desc_state[id];
  1442	
  1443		/* Clear data ptr. */
  1444		state->data = NULL;
  1445	
  1446		vq->packed.desc_extra[state->last].next = vq->free_head;
  1447		vq->free_head = id;
  1448		vq->vq.num_free += state->num;
  1449	
  1450		if (unlikely(vq->use_dma_api)) {
  1451			curr = id;
  1452			for (i = 0; i < state->num; i++) {
  1453				vring_unmap_state_packed(vq,
  1454					&vq->packed.desc_extra[curr]);
  1455				curr = vq->packed.desc_extra[curr].next;
  1456			}
  1457		}
  1458	
  1459		if (vq->indirect) {
  1460			u32 len, n;
  1461	
  1462			/* Free the indirect table, if any, now that it's unmapped. */
  1463			desc = state->indir_desc;
  1464			if (!desc)
  1465				return;
  1466	

1467			n = len / sizeof(struct vring_packed_desc);

  1468	
  1469			if (vq->use_dma_api) {
  1470				len = vq->packed.desc_extra[id].len;
  1471				for (i = 0; i < n; i++)
  1472					vring_unmap_desc_packed(vq, &desc[i]);
  1473			}
  1474	
  1475			desc_cache_put_packed(vq, desc, n);
  1476			state->indir_desc = NULL;
  1477		} else if (ctx) {
  1478			*ctx = state->indir_desc;
  1479		}
  1480	}
  1481	

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org