Re: [PATCH] IOMMU and ATS not supported by vhost-user filesystem.

[Dr. David Alan Gilbert]

* Laszlo Ersek (lersek@redhat.com) wrote:
> On 01/27/21 12:19, Stefan Hajnoczi wrote:
> > On Tue, Jan 26, 2021 at 03:23:38PM -0300, lagarcia@linux.ibm.com wrote:
> >> From: Leonardo Garcia <lagarcia@br.ibm.com>
> >>
> >> Currently, as IOMMU and ATS are not supported, if a user mistakenly set
> >> any of them and tries to mount the vhost-user filesystem inside the
> >> guest, whenever the user tries to access the mount point, the system
> >> will hang forever.
> >>
> >> Signed-off-by: Leonardo Garcia <lagarcia@br.ibm.com>
> >> ---
> >>  hw/virtio/vhost-user-fs-pci.c | 7 +++++++
> >>  hw/virtio/vhost-user-fs.c     | 5 +++++
> >>  2 files changed, 12 insertions(+)
> >>
> >> diff --git a/hw/virtio/vhost-user-fs-pci.c b/hw/virtio/vhost-user-fs-pci.c
> >> index 2ed8492b3f..564d1fd108 100644
> >> --- a/hw/virtio/vhost-user-fs-pci.c
> >> +++ b/hw/virtio/vhost-user-fs-pci.c
> >> @@ -11,6 +11,8 @@
> >>   * top-level directory.
> >>   */
> >>  
> >> +#include "qemu/osdep.h"
> >> +#include "qapi/error.h"
> >>  #include "qemu/osdep.h"
> >>  #include "hw/qdev-properties.h"
> >>  #include "hw/virtio/vhost-user-fs.h"
> >> @@ -45,6 +47,11 @@ static void vhost_user_fs_pci_realize(VirtIOPCIProxy 
> >> *vpci_dev, Error **errp)
> >>          vpci_dev->nvectors = dev->vdev.conf.num_request_queues + 2;
> >>      }
> >>  
> >> +    if (vpci_dev->flags & VIRTIO_PCI_FLAG_ATS) {
> >> +        error_setg(errp, "ATS is currently not supported with 
> >> vhost-user-fs-pci");
> >> +        return;
> >> +    }
> > 
> > Why is this check needed in addition to VIRTIO_F_IOMMU_PLATFORM?
> > 
> > What needs to be added to support ATS?
> > 
> >> +
> >>      qdev_realize(vdev, BUS(&vpci_dev->bus), errp);
> >>  }
> >>  
> >> diff --git a/hw/virtio/vhost-user-fs.c b/hw/virtio/vhost-user-fs.c
> >> index ac4fc34b36..914d68b3ee 100644
> >> --- a/hw/virtio/vhost-user-fs.c
> >> +++ b/hw/virtio/vhost-user-fs.c
> >> @@ -203,6 +203,11 @@ static void vuf_device_realize(DeviceState *dev, 
> >> Error **errp)
> >>          return;
> >>      }
> >>  
> >> +    if (virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM)) {
> >> +        error_setg(errp, "IOMMU is currently not supported with 
> >> vhost-user-fs");
> >> +        return;
> >> +    }
> >> +
> >>      if (!vhost_user_init(&fs->vhost_user, &fs->conf.chardev, errp)) {
> > 
> > I thought IOMMU support depends on the vhost-user device backend (e.g.
> > virtiofsd), so the vhost-user backend should participate in advertising
> > this feature.
> 
> ... I had the same thought when a few weeks earlier I tried to use
> virtio-fs with an SEV guest (just OVMF), and virtiofsd crashed,
> apparently :)
> 
> (I didn't report it because, well, SEV wants to prevent sharing between
> host and guest, and virtio-fs works precisely in the opposite direction;
> so the use case may not have much merit.)

Yeh I'm not expecting that to currently work; I can see some uses, but
it's much more niche.

Dave

> Thanks
> Laszlo
> 
> > 
> > Perhaps the check should be:
> > 
> >     ret = vhost_dev_init(&fs->vhost_dev, &fs->vhost_user,
> >                          VHOST_BACKEND_TYPE_USER, 0);
> >     if (ret < 0) {
> >         error_setg_errno(errp, -ret, "vhost_dev_init failed");
> >         goto err_virtio;
> >     }
> > +
> > +   if (virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM) &&
> > +       !(fs->vhost_dev.hdev_features & (1ull << VIRTIO_F_IOMMU_PLATFORM))) 
> > {
> > +       error_setg(errp, "IOMMU is not supported by the vhost-user device 
> > backend");
> > +       goto err_iommu_needed;
> > +   }
> > 
> > Also, can this logic be made generic for all vhost-user devices? It's
> > not really specific to vhost-user-fs.
> > 
> > Stefan
> > 
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK