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

[Peter Xu <peterx@redhat.com>]

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,

-- 
Peter Xu