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

Stefan Hajnoczi
2026-01-07T16:29:54Z

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On Tue, Jan 06, 2026 at 09:50:00AM -0500, Michael S. Tsirkin wrote:
> 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?
>
> No because virtio does unmap and syncs the cache line.
>
> In other words, CPU reads cause no issues.
>
> The issues are exclusively around CPU writes dirtying the
> cache and writeback overwriting DMA data.

I see. In that case I'm happy with the virtio-scsi change:

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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