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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 6 Jan 2026 09:50:00 -0500

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.

> 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