linux-kernel - Re: [PATCH v2 1/2] virtio: silence KCSAN warning in virtqueue_get_buf_ctx

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Message-ID: <4fde2f6e-d272-4aa8-9701-63bc6b7e7d16@amazon.com>
Date: Wed, 28 Jan 2026 10:03:26 +0100
From: Alexander Graf <graf@...zon.com>
To: Johannes Thumshirn <Johannes.Thumshirn@....com>,
	"virtualization@...ts.linux.dev" <virtualization@...ts.linux.dev>
CC: "Michael S . Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez
	<eperezma@...hat.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] virtio: silence KCSAN warning in
 virtqueue_get_buf_ctx_split

On 28.01.26 09:47, Johannes Thumshirn wrote:
> On 1/27/26 5:30 PM, Alexander Graf wrote:
>> This patches the split vring format, but does not touch the packed one.
>> What happens if you run the same test with the packed format? You can do
>> so by passing "packed=on" as argument to your -device parameter.
> This opened up a whole new can of worms... :(

That's what I expected :).

How do other DMA based devices handle this? Is the real problem that 
virtio by default does not use the DMA API and so it confuses generic 
KCSAN logic that would otherwise track DMA regions as "can be modified 
by DMA at any time"?

If that is the case, maybe what we really want is to force enable use of 
the DMA API when KCSAN is active. Does something like the (whitespace 
broken) patch below work?

Alex

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index ddab68959671..b1dd790ce622 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -284,6 +284,13 @@ static bool vring_use_map_api(const struct 
virtio_device *vdev)
      if (xen_domain())
          return true;

+    /*
+     * KCSAN needs to track who can modify memory. DMA API gets
+     * us that, so always use it.
+     */
+    if (IS_ENABLED(CONFIG_KCSAN))
+        return true;
+
      return false;
  }

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