Re: [PATCH v2 1/2] virtio: silence KCSAN warning in virtqueue_get_buf_ctx_split

[Alexander Graf <graf@amazon.com>]

On 28.01.26 10:13, Johannes Thumshirn wrote:
> On 1/28/26 10:03 AM, Alexander Graf wrote:
>> 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;
>>     }
>
> Unfortunately this doesn't get us any further (I'd love though, it looks
> way cleaner!)
>
> I still see the KCSAN messages even on boot.


Ah, looks like the important bit for KCSAN is not the mapping mechanism, 
it's the actual compiler annotation for the read. So these virtio ring 
reads should all be annotated as READ_ONCE() to make sure KCSAN knows 
the read itself is atomic.

Alex




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