Re: [PATCH V2] vhost: correctly turn on VIRTIO_F_IOMMU_PLATFORM

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

On Wed, Feb 26, 2020 at 04:36:18PM +0100, Halil Pasic wrote:
> On Wed, 26 Feb 2020 08:37:13 -0500
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Wed, Feb 26, 2020 at 02:28:39PM +0100, Halil Pasic wrote:
> > > On Wed, 26 Feb 2020 17:43:57 +0800
> > > Jason Wang <jasowang@redhat.com> wrote:
> > > 
> > > > We turn on device IOTLB via VIRTIO_F_IOMMU_PLATFORM unconditionally on
> > > > platform without IOMMU support. This can lead unnecessary IOTLB
> > > > transactions which will damage the performance.
> > > > 
> > > > Fixing this by check whether the device is backed by IOMMU and disable
> > > > device IOTLB.
> > > > 
> > > > Reported-by: Halil Pasic <pasic@linux.ibm.com>
> > > > Fixes: c471ad0e9bd46 ("vhost_net: device IOTLB support")
> > > > Cc: qemu-stable@nongnu.org
> > > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > > 
> > > Tested-by: Halil Pasic <pasic@linux.ibm.com>
> > > Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
> > > 
> > > Thank you very much for fixing this! BTW as I mentioned before it
> > > fixes vhost-vsock with iommu_platform=on as well.
> > 
> > Fixes as in improves performance?
> 
> No, fixes like one does not get something like:
> qemu-system-s390x: vhost_set_features failed: Operation not supported (95)
> qemu-system-s390x: Error starting vhost: 95
> any more.
> 
> Regards,
> Halil
> 
> [..]

But can commit c471ad0e9bd46 actually boot a secure guest
where iommu_platform=on is required?

-- 
MST