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

Michael S. Tsirkin
2026-01-08T14:08:01Z

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?

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