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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Mar 26, 2026 at 06:17:36PM +0000, Simon Horman wrote:
> On Tue, Mar 24, 2026 at 11:15:54AM -0400, Michael S. Tsirkin 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")
> 
> Hi Michael,
> 
> The hash for a patch with that subject in net-next appears to be 24fbd3967f3f.

Thanks, will fix in the next version.

> > 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>
> 
> ...