On Wed, Feb 26, 2020 at 02:28:39PM +0100, Halil Pasic wrote:
> On Wed, 26 Feb 2020 17:43:57 +0800
> Jason Wang <jasow...@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 <pa...@linux.ibm.com>
> > Fixes: c471ad0e9bd46 ("vhost_net: device IOTLB support")
> > Cc: qemu-sta...@nongnu.org
> > Signed-off-by: Jason Wang <jasow...@redhat.com>
>
> Tested-by: Halil Pasic <pa...@linux.ibm.com>
> Reviewed-by: Halil Pasic <pa...@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?
> Regards,
> Halil
>
> > ---
> > Changes from V1:
> > - do not check acked_features
> > - reuse vhost_dev_has_iommu()
> > ---
> > 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..9182a00495 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_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
> > }
> >
> > static void *vhost_memory_map(struct vhost_dev *dev, hwaddr addr,
> > @@ -765,6 +772,9 @@ static int vhost_dev_set_features(struct vhost_dev *dev,
> > if (enable_log) {
> > features |= 0x1ULL << VHOST_F_LOG_ALL;
> > }
> > + if (!vhost_dev_has_iommu(dev)) {
> > + features &= ~(0x1ULL << VIRTIO_F_IOMMU_PLATFORM);
> > + }
> > r = dev->vhost_ops->vhost_set_features(dev, features);
> > if (r < 0) {
> > VHOST_OPS_DEBUG("vhost_set_features failed");
