On Wed, Sep 04, 2024 at 03:30:02PM +0800, Xuan Zhuo wrote:

On Wed, 4 Sep 2024 15:21:28 +0800, =?utf-8?b?5paH5Y2a5p2O?= [off-list ref] wrote:

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When SWIOTLB is enabled, a DMA map will allocate a bounce buffer for real
DMA operations,
and when unmapping, SWIOTLB copies the content in the bounce buffer to the
driver-allocated
buffer (the `buf` variable). Such copy only synchronizes data in the buffer
range, not the whole page.
So we should give the correct offset for DMA unmapping.

I see.

But I think we should pass the "correct" buf to virtio core as the "data" by
virtqueue_add_inbuf_ctx().

In the merge mode, we pass the pointer that points to the virtnet header.
In the small mode, we pass the pointer that points to the virtnet header - offset.

But this is not the only problem, we try to get the virtnet header by the buf
inside receive_buf(before receive_small).

	flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags;

So I think it is time to unify the buf that passed to the virtio core into a
pointer pointed to the virtnet header.

Thanks.

Hard to say without seeing what is proposed, but I think
data pointer is opaque to virtio, it does not have to
point to anything specific - just !NULL.

quoted

Thanks.

On Wed, Sep 4, 2024 at 2:46 PM Xuan Zhuo [off-list ref] wrote:

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On Wed,  4 Sep 2024 14:10:09 +0800, Wenbo Li [off-list ref]

wrote:

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Currently, the virtio-net driver will perform a pre-dma-mapping for
small or mergeable RX buffer. But for small packets, a mismatched

address

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without VIRTNET_RX_PAD and xdp_headroom is used for unmapping.

Will used virt_to_head_page(), so could you say more about it?

        struct page *page = virt_to_head_page(buf);

Thanks.

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That will result in unsynchronized buffers when SWIOTLB is enabled, for
example, when running as a TDX guest.

This patch handles small and mergeable packets separately and fixes
the mismatched buffer address.

Changes from v1: Use ctx to get xdp_headroom.

Fixes: 295525e29a5b ("virtio_net: merge dma operations when filling

mergeable buffers")

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Signed-off-by: Wenbo Li <redacted>
Signed-off-by: Jiahui Cen <redacted>
Signed-off-by: Ying Fang <redacted>
---
 drivers/net/virtio_net.c | 29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index c6af18948..cbc3c0ae4 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -891,6 +891,23 @@ static void *virtnet_rq_get_buf(struct

receive_queue *rq, u32 *len, void **ctx)

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      return buf;
 }

+static void *virtnet_rq_get_buf_small(struct receive_queue *rq,
+                                   u32 *len,
+                                   void **ctx,
+                                   unsigned int header_offset)
+{
+     void *buf;
+     unsigned int xdp_headroom;
+
+     buf = virtqueue_get_buf_ctx(rq->vq, len, ctx);
+     if (buf) {
+             xdp_headroom = (unsigned long)*ctx;
+             virtnet_rq_unmap(rq, buf + VIRTNET_RX_PAD + xdp_headroom,

*len);

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+     }
+
+     return buf;
+}
+
 static void virtnet_rq_init_one_sg(struct receive_queue *rq, void

*buf, u32 len)

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 {
      struct virtnet_rq_dma *dma;

@@ -2692,13 +2709,23 @@ static int virtnet_receive_packets(struct

virtnet_info *vi,

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      int packets = 0;
      void *buf;

-     if (!vi->big_packets || vi->mergeable_rx_bufs) {
+     if (vi->mergeable_rx_bufs) {
              void *ctx;
              while (packets < budget &&
                     (buf = virtnet_rq_get_buf(rq, &len, &ctx))) {
                      receive_buf(vi, rq, buf, len, ctx, xdp_xmit,

stats);

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                      packets++;
              }
+     } else if (!vi->big_packets) {
+             void *ctx;
+             unsigned int xdp_headroom = virtnet_get_headroom(vi);
+             unsigned int header_offset = VIRTNET_RX_PAD +

xdp_headroom;

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+
+             while (packets < budget &&
+                    (buf = virtnet_rq_get_buf_small(rq, &len, &ctx,

header_offset))) {

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+                     receive_buf(vi, rq, buf, len, ctx, xdp_xmit,

stats);

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+                     packets++;
+             }
      } else {
              while (packets < budget &&
                     (buf = virtqueue_get_buf(rq->vq, &len)) != NULL) {
--
2.20.1