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 09:51:46 -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?
> 
> 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.

-- 
MST