Re: [PATCH] vhost: correctly turn on VIRTIO_F_IOMMU_PLATFORM

[Jason Wang <jasowang@redhat.com>]

----- Original Message -----
> On Wed, Feb 26, 2020 at 03:06:47PM +0800, Jason Wang 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")
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > ---
> >  hw/virtio/vhost.c | 12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> > 
> > diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> > index 9edfadc81d..6e12c3d2de 100644
> > --- a/hw/virtio/vhost.c
> > +++ b/hw/virtio/vhost.c
> > @@ -290,7 +290,14 @@ static int vhost_dev_has_iommu(struct vhost_dev *dev)
> >  {
> >      VirtIODevice *vdev = dev->vdev;
> >  
> > -    return virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
> > +    /*
> > +     * For vhost, VIRTIO_F_IOMMU_PLATFORM means the backend support
> > +     * incremental memory mapping API via IOTLB API. For platform that
> > +     * does not have IOMMU, there's no need to enable this feature
> > +     * which may cause unnecessary IOTLB miss/update trnasactions.
> > +     */
> > +    return vdev->dma_as != &address_space_memory &&
> > +           virtio_has_feature(dev->acked_features,
> > VIRTIO_F_IOMMU_PLATFORM);
> >  }
> >  
> >  static void *vhost_memory_map(struct vhost_dev *dev, hwaddr addr,
> 
> Why check acked_features and not host features here?
> I'd worry that if we do it like this, userspace driver
> within guest can clear the feature and make device access
> memory directly.

Right, host_features should be more than enough.

> 
> > @@ -765,6 +772,9 @@ static int vhost_dev_set_features(struct vhost_dev
> > *dev,
> >      if (enable_log) {
> >          features |= 0x1ULL << VHOST_F_LOG_ALL;
> >      }
> > +    if (dev->vdev->dma_as == &address_space_memory) {
> > +        features &= ~(0x1ULL << VIRTIO_F_IOMMU_PLATFORM);
> > +    }
> 
> 
> That's a guest visible change. Which seems at best unnecessary.
>

I don't get how this can be visible from guest? It works as F_LOG_ALL.

Thanks

> >      r = dev->vhost_ops->vhost_set_features(dev, features);
> >      if (r < 0) {
> >          VHOST_OPS_DEBUG("vhost_set_features failed");
> > --
> > 2.19.1
> 
>