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Thu, 26 Mar 2026 15:03:32 -0700 (PDT) Date: Thu, 26 Mar 2026 18:03:28 -0400 From: "Michael S. Tsirkin" To: Simon Horman Cc: linux-kernel@vger.kernel.org, Omar Elghoul , Srikanth Aithal , Jason Wang , Xuan Zhuo , Eugenio =?iso-8859-1?Q?P=E9rez?= , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , Stanislav Fomichev , Vishwanath Seshagiri , netdev@vger.kernel.org, virtualization@lists.linux.dev, bpf@vger.kernel.org Subject: Re: [PATCH net-next] virtio_net: sync RX buffer before reading the header Message-ID: <20260326180313-mutt-send-email-mst@kernel.org> References: <20260326181736.GV111839@horms.kernel.org> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260326181736.GV111839@horms.kernel.org> 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 > > Tested-by: Srikanth Aithal > > Tested-by: Omar Elghoul > > Link: https://lore.kernel.org/r/20260323150136.14452-1-oelghoul@linux.ibm.com > > Signed-off-by: Michael S. Tsirkin > > ...