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

From: Stefan Hajnoczi <[email-masked]>
Date: Wed, 7 Jan 2026 11:29:39 -0500

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 <[email-masked]>