Re: [RFC v2 5/7] virtio: unmask F_NEXT flag in desc_extra

----- Original Message -----

From: "Xuan Zhuo" <xuanzhuo@linux.alibaba.com>
To: "Guo Zhi" <redacted>
Cc: "netdev" <redacted>, "linux-kernel" <redacted>, "kvm list" <redacted>,
"virtualization" [off-list ref], "Guo Zhi" [off-list ref], "eperezma"
[off-list ref], "jasowang" [off-list ref], "sgarzare" [off-list ref], "Michael Tsirkin"
[off-list ref]
Sent: Thursday, August 18, 2022 11:05:37 AM
Subject: Re: [RFC v2 5/7] virtio: unmask F_NEXT flag in desc_extra

On Wed, 17 Aug 2022 21:57:16 +0800, Guo Zhi [off-list ref] wrote:

quoted

We didn't unmask F_NEXT flag in desc_extra in the end of a chain,
unmask it so that we can access desc_extra to get next information.

I think we should state the purpose of this.

I have to unmask F_NEXT flag in desc_extra in this series, because if in order
negotiated, the driver has to iterate the descriptor chain to get chain length
from desc_extra. (The reason why we should use desc_extra is that descs may be
changed by malicious hypervisors, https://lkml.org/lkml/2022/7/26/224).

quoted

Signed-off-by: Guo Zhi <redacted>
---
 drivers/virtio/virtio_ring.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index a5ec724c01d8..1c1b3fa376a2 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c

@@ -567,7 +567,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
 	}
 	/* Last one doesn't continue. */
 	desc[prev].flags &= cpu_to_virtio16(_vq->vdev, ~VRING_DESC_F_NEXT);
-	if (!indirect && vq->use_dma_api)
+	if (!indirect)
 		vq->split.desc_extra[prev & (vq->split.vring.num - 1)].flags &=
 			~VRING_DESC_F_NEXT;

@@ -584,6 +584,8 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
 					 total_sg * sizeof(struct vring_desc),
 					 VRING_DESC_F_INDIRECT,
 					 false);
+		vq->split.desc_extra[head & (vq->split.vring.num - 1)].flags &=
+			~VRING_DESC_F_NEXT;

This seems unnecessary.

quoted

 	}

 	/* We're using some buffers from the free list. */

@@ -693,7 +695,7 @@ static void detach_buf_split(struct vring_virtqueue *vq,

unsigned int head,
 	/* Put back on free list: unmap first-level descriptors and find end */
 	i = head;

-	while (vq->split.vring.desc[i].flags & nextflag) {
+	while (vq->split.desc_extra[i].flags & nextflag) {

nextflag is __virtio16

You can use VRING_DESC_F_NEXT directly.

Thanks.

Sorry for the mistake, I will fix it.

quoted

 		vring_unmap_one_split(vq, i);
 		i = vq->split.desc_extra[i].next;
 		vq->vq.num_free++;
--
2.17.1