Re: [Qemu-devel] [PATCH 2/3] virtio: access ISR atomically

[Paolo Bonzini <pbonzini@redhat.com>]

----- Original Message -----
> From: "Michael S. Tsirkin" <mst@redhat.com>
> To: "Paolo Bonzini" <pbonzini@redhat.com>
> Cc: qemu-devel@nongnu.org, "alex williamson" <alex.williamson@redhat.com>, borntraeger@de.ibm.com, felipe@nutanix.com
> Sent: Thursday, November 17, 2016 6:55:50 PM
> Subject: Re: [PATCH 2/3] virtio: access ISR atomically
> 
> On Wed, Nov 16, 2016 at 07:05:50PM +0100, Paolo Bonzini wrote:
> > @@ -1318,10 +1318,18 @@ void virtio_del_queue(VirtIODevice *vdev, int n)
> >      vdev->vq[n].vring.num_default = 0;
> >  }
> >  
> > +static void virtio_set_isr(VirtIODevice *vdev, int value)
> > +{
> > +    uint8_t old = atomic_read(&vdev->isr);
> > +    if ((old & value) != value) {
> > +        atomic_or(&vdev->isr, value);
> > +    }
> > +}
> > +
> 
> Paolo, can you pls comment on why is it done like this,
> as opposed to a single atomic_or?

To avoid cacheline bouncing in the common case where MSI is active
but the host doesn't read ISR.

Paolo