Re: [PATCH v2 10/15] virtio_scsi: fix DMA cacheline issues for events

Michael S. Tsirkin
2026-01-06T14:51:55Z

On Mon, Jan 05, 2026 at 01:19:39PM -0500, Stefan Hajnoczi wrote:
> On Mon, Jan 05, 2026 at 03:23:29AM -0500, Michael S. Tsirkin wrote:
> > @@ -61,7 +62,7 @@ struct virtio_scsi_cmd {
> >
> > struct virtio_scsi_event_node {
> > struct virtio_scsi *vscsi;
> > - struct virtio_scsi_event event;
> > + struct virtio_scsi_event *event;
> > struct work_struct work;
> > };
> >
> > @@ -89,6 +90,11 @@ struct virtio_scsi {
> >
> > struct virtio_scsi_vq ctrl_vq;
> > struct virtio_scsi_vq event_vq;
> > +
> > + __dma_from_device_group_begin();
> > + struct virtio_scsi_event events[VIRTIO_SCSI_EVENT_LEN];
> > + __dma_from_device_group_end();
>
> If the device emits two events in rapid succession, could the CPU see
> stale data for the second event because it already holds the cache line
> for reading the first event?
>
> In other words, it's not obvious to me that the DMA warnings are indeed
> spurious and should be silenced here.
>
> It seems safer and simpler to align and pad the struct virtio_scsi_event
> field in struct virtio_scsi_event_node rather than packing these structs
> into a single array here they might share cache lines.
>
> Stefan

To add to what I wrote, that's a lot of overhead: 8 * 128 - about 1K on
some platforms, and these happen to be low end ones.

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MST