Re: [PATCH v2 07/15] vsock/virtio: fix DMA alignment for event_list

Stefano Garzarella
2026-01-08T14:18:26Z

On Thu, Jan 08, 2026 at 09:07:53AM -0500, Michael S. Tsirkin wrote:
>On Thu, Jan 08, 2026 at 03:04:07PM +0100, Stefano Garzarella wrote:
>> On Mon, Jan 05, 2026 at 03:23:17AM -0500, Michael S. Tsirkin wrote:
>> > On non-cache-coherent platforms, when a structure contains a buffer
>> > used for DMA alongside fields that the CPU writes to, cacheline sharing
>> > can cause data corruption.
>> >
>> > The event_list array is used for DMA_FROM_DEVICE operations via
>> > virtqueue_add_inbuf(). The adjacent event_run and guest_cid fields are
>> > written by the CPU while the buffer is available, so mapped for the
>> > device. If these share cachelines with event_list, CPU writes can
>> > corrupt DMA data.
>> >
>> > Add __dma_from_device_group_begin()/end() annotations to ensure event_list
>> > is isolated in its own cachelines.
>> >
>> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>> > ---
>> > net/vmw_vsock/virtio_transport.c | 4 +++-
>> > 1 file changed, 3 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
>> > index 8c867023a2e5..bb94baadfd8b 100644
>> > --- a/net/vmw_vsock/virtio_transport.c
>> > +++ b/net/vmw_vsock/virtio_transport.c
>> > @@ -17,6 +17,7 @@
>> > #include <linux/virtio_ids.h>
>> > #include <linux/virtio_config.h>
>> > #include <linux/virtio_vsock.h>
>> > +#include <linux/dma-mapping.h>
>> > #include <net/sock.h>
>> > #include <linux/mutex.h>
>> > #include <net/af_vsock.h>
>> > @@ -59,8 +60,9 @@ struct virtio_vsock {
>> > */
>> > struct mutex event_lock;
>> > bool event_run;
>> > + __dma_from_device_group_begin();
>> > struct virtio_vsock_event event_list[8];
>> > -
>> > + __dma_from_device_group_end();
>>
>> Can we keep the blank line before `guest_cid` so that the comment before
>> this section makes sense? (regarding the lock required to access these
>> fields)
>>
>> Thanks,
>> Stefano
>
>A follow up patch re-introduces it, so I don't think it matters?

Yes, I saw it later. Of course I don't want you to resend the whole
series just for this. So if you have to resend the series for other
reasons, I would avoid removing the line here because I don't see any
value on removing it and add back later.

In both cases:

Acked-by: Stefano Garzarella <sgarzare@redhat.com>

>
>> > u32 guest_cid;
>> > bool seqpacket_allow;
>> >
>> > --
>> > MST
>> >
>