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Date: Thu, 26 Mar 2026 18:17:36 +0000
From: Simon Horman <horms@kernel.org>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: linux-kernel@vger.kernel.org, Omar Elghoul <oelghoul@linux.ibm.com>,
	Srikanth Aithal <sraithal@amd.com>,
	Jason Wang <jasowang@redhat.com>,
	Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
	Eugenio =?utf-8?B?UMOpcmV6?= <eperezma@redhat.com>,
	Andrew Lunn <andrew+netdev@lunn.ch>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	John Fastabend <john.fastabend@gmail.com>,
	Stanislav Fomichev <sdf@fomichev.me>,
	Vishwanath Seshagiri <vishs@meta.com>, 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: <20260326181736.GV111839@horms.kernel.org>
References: <f4caa9be9e5addae7851c012cab0a733be7f0974.1774365273.git.mst@redhat.com>
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In-Reply-To: <f4caa9be9e5addae7851c012cab0a733be7f0974.1774365273.git.mst@redhat.com>

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.

> 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>

...