----- 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 <pa...@linux.ibm.com>
> > Fixes: c471ad0e9bd46 ("vhost_net: device IOTLB support")
> > Signed-off-by: Jason Wang <jasow...@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
>
>
