Re: [PATCH v3 1/1] virtio: fix the condition for iommu_platform not supported

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

On Tue, Feb 01, 2022 at 05:47:24PM +0100, Cornelia Huck wrote:
> On Tue, Feb 01 2022, Halil Pasic <pasic@linux.ibm.com> wrote:
> 
> > The commit 04ceb61a40 ("virtio: Fail if iommu_platform is requested, but
> > unsupported") claims to fail the device hotplug when iommu_platform
> > is requested, but not supported by the (vhost) device. On the first
> > glance the condition for detecting that situation looks perfect, but
> > because a certain peculiarity of virtio_platform it ain't.
> >
> > In fact the aforementioned commit introduces a regression. It breaks
> > virtio-fs support for Secure Execution, and most likely also for AMD SEV
> > or any other confidential guest scenario that relies encrypted guest
> > memory.  The same also applies to any other vhost device that does not
> > support _F_ACCESS_PLATFORM.
> >
> > The peculiarity is that iommu_platform and _F_ACCESS_PLATFORM collates
> > "device can not access all of the guest RAM" and "iova != gpa, thus
> > device needs to translate iova".
> >
> > Confidential guest technologies currently rely on the device/hypervisor
> > offering _F_ACCESS_PLATFORM, so that, after the feature has been
> > negotiated, the guest  grants access to the portions of memory the
> > device needs to see. So in for confidential guests, generally,
> > _F_ACCESS_PLATFORM is about the restricted access to memory, but not
> > about the addresses used being something else than guest physical
> > addresses.
> >
> > This is the very reason for which commit f7ef7e6e3b ("vhost: correctly
> > turn on VIRTIO_F_IOMMU_PLATFORM") for, which fences _F_ACCESS_PLATFORM
> 
> s/for, which //
> 
> > form the vhost device that does not need it, because on the vhost
> 
> s/form/from/
> 
> > interface it only means "I/O address translation is needed".
> >
> > This patch takes inspiration from f7ef7e6e3b ("vhost: correctly turn on
> > VIRTIO_F_IOMMU_PLATFORM"), and uses the same condition for detecting the
> > situation when _F_ACCESS_PLATFORM is requested, but no I/O translation
> > by the device, and thus no device capability is needed. In this
> > situation claiming that the device does not support iommu_plattform=on
> > is counter-productive. So let us stop doing that!
> >
> > Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
> > Reported-by: Jakob Naucke <Jakob.Naucke@ibm.com>
> > Fixes: 04ceb61a40 ("virtio: Fail if iommu_platform is requested, but
> > unsupported")
> > Cc: Kevin Wolf <kwolf@redhat.com>
> > Cc: qemu-stable@nongnu.org
> >
> > ---
> >
> > v2->v3:
> > * Caught a bug: I tired to check if vdev has the feature
> >    ACCESS_PLATFORM after we have forced it. Moved the check
> >    to a better place
> > v1->v2:
> > * Commit message tweaks. Most notably fixed commit SHA (Michael)
> >
> > ---
> > ---
> >  hw/virtio/virtio-bus.c | 11 ++++++-----
> >  1 file changed, 6 insertions(+), 5 deletions(-)
> >
> > diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
> > index d23db98c56..34f5a0a664 100644
> > --- a/hw/virtio/virtio-bus.c
> > +++ b/hw/virtio/virtio-bus.c
> > @@ -48,6 +48,7 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp)
> >      VirtioBusClass *klass = VIRTIO_BUS_GET_CLASS(bus);
> >      VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(vdev);
> >      bool has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
> > +    bool vdev_has_iommu = false;
> 
> Isn't vdev_has_iommu set unconditionally before you try to use it?

I'd like to know too.

> >      Error *local_err = NULL;
> >  
> >      DPRINTF("%s: plug device.\n", qbus->name);
> > @@ -69,11 +70,6 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp)
> >          return;
> >      }
> >  
> > -    if (has_iommu && !virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM)) {
> > -        error_setg(errp, "iommu_platform=true is not supported by the device");
> > -        return;
> > -    }
> > -
> >      if (klass->device_plugged != NULL) {
> >          klass->device_plugged(qbus->parent, &local_err);
> >      }
> > @@ -82,9 +78,14 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp)
> >          return;
> >      }
> >  
> > +    vdev_has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
> >      if (klass->get_dma_as != NULL && has_iommu) {
> >          virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
> >          vdev->dma_as = klass->get_dma_as(qbus->parent);
> > +        if (!vdev_has_iommu && vdev->dma_as != &address_space_memory) {
> > +            error_setg(errp,
> > +                       "iommu_platform=true is not supported by the device");
> > +        }
> >      } else {
> 
> I agree that a short comment would be nice here, but this is preexisting
> code anyway...
> 
> >          vdev->dma_as = &address_space_memory;
> >      }
> >
> > base-commit: 6621441db50d5bae7e34dbd04bf3c57a27a71b32
> 
> ...so (with or without fixing the nits):
> 
> Acked-by: Cornelia Huck <cohuck@redhat.com>
> 
> (i.e. looks sane, but I didn't follow all the paths)