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

[Stefan Hajnoczi]

On Wed, Jan 27, 2021 at 09:30:35AM -0300, Leonardo Augusto Guimarães Garcia 
wrote:
> On 1/27/21 8:19 AM, 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?
> 
> 
> I don't know if VIRTIO_PCI_FLAG_ATS should depend on
> VIRTIO_F_IOMMU_PLATFORM. At least from a code perspective today, they are
> completely independent. A user can specify one or the other or both. And if
> a user specifies VIRTIO_PCI_FLAG_ATS without specifying
> VIRTIO_F_IOMMU_PLATFORM, the same issue described in the original message
> will happen inside the guest.

I don't see any PCI ATS-specific code in Linux drivers/virtio/ so I
wonder what the issue is?

Also, I thought PCI ATS is an optional feature. It's still possible to
have IOMMUs without ATS.

Michael: Can you help us understand why enabling ATS on a device in QEMU
would cause issues with a VIRTIO device that does not support
VIRTIO_F_IOMMU_PLATFORM?

> > 
> > > +
> > >       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.
> > 
> > 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.
> 
> 
> Sure, I can do that. I wasn't sure whether this restriction was only for
> vhost-user-fs or whether it was generic for all vhost-user devices. I will
> include this in a next version of the patch.

Awesome, thanks!

Stefan

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