Re: [PATCH net-next] virtio_net: sync RX buffer before reading the header

[Jason Wang <jasowang@redhat.com>]

On Tue, Mar 24, 2026 at 11:16 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> receive_buf() reads the virtio header through buf before
> page_pool_dma_sync_for_cpu() runs in receive_small() or
> receive_mergeable(). The header buffer is thus unsynchronized at the
> point where flags and, for mergeable buffers, num_buffers are consumed.
>
> Omar Elghoul reported that on s390x Secure Execution this showed up as
> greatly reduced virtio-net performance together with "bad gso" and
> "bad csum" messages in dmesg. This is because with SE sync actually
> copies data, so the header is uninitialized.
>
> Move the sync into receive_buf() so the
> header is synchronized before any access through buf.
>
> Tool use: Cursor with GPT-5.4 drafted the initial code move from prompt:
> "in drivers/net/virtio_net.c, move page_pool_dma_sync_for_cpu on receive
> path to before memory is accessed through buf".
> The result and the commit log were reviewed and edited manually.
>
> Fixes: 168b61da6871 ("virtio_net: add page_pool support for buffer allocation")
> Reported-by: Omar Elghoul <oelghoul@linux.ibm.com>
> Tested-by: Srikanth Aithal <sraithal@amd.com>
> Tested-by: Omar Elghoul <oelghoul@linux.ibm.com>
> Link: https://lore.kernel.org/r/20260323150136.14452-1-oelghoul@linux.ibm.com
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Acked-by: Jason Wang <jasowang@redhat.com>

Thanks