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

Stefano Garzarella
2026-01-08T14:04:30Z

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

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