Re: [Qemu-devel] [PATCH 3/3] vhost: iommu: cache static mapping if there is

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jun 05, 2017 at 11:15:11AM +0800, Peter Xu wrote:
> On Fri, Jun 02, 2017 at 06:45:05PM +0300, Michael S. Tsirkin wrote:
> > On Fri, Jun 02, 2017 at 07:50:54PM +0800, Peter Xu wrote:
> > > This patch pre-heat vhost iotlb cache when passthrough mode enabled.
> > > 
> > > Sometimes, even if user specified iommu_platform for vhost devices,
> > > IOMMU might still be disabled. One case is passthrough mode in VT-d
> > > implementation. We can detect this by observing iommu_list. If it's
> > > empty, it means IOMMU translation is disabled, then we can actually
> > > pre-heat the translation (it'll be static mapping then) by first
> > > invalidating all IOTLB, then cache existing memory ranges into vhost
> > > backend iotlb using 1:1 mapping.
> > > 
> > > Reviewed-by: Jason Wang <jasowang@redhat.com>
> > > Signed-off-by: Peter Xu <peterx@redhat.com>
> > 
> > This is still a hack I think. What if there's an invalidation?
> > I think the right thing is to send updates only when requested,
> > but sent the largest mapping including the iova, not from iova until end
> > of page. Thoughts?
> 
> Indeed it's kind of a hack, but it does not hurt anything but will
> definitely boost performance in most cases...
> 
> Yes "sent the largest mapping including the iova" is okay, but the
> first IO on one region would be delayed as well, so IMHO it's not the
> best solution as well. I think the best solution should be (for sure)
> that vhost knows it's PT, then it just skips the translation
> completely. I just don't sure whether there's simple/good way to do
> this.
> 
> Thanks,

If you send the whole 64 bit area, then backend can detect this
easily.


> -- 
> Peter Xu