Re: [RFC PATCH 3/3] kvm: Add VFIO device for handling IOMMU cache coherency

Alex Williamson
2013-09-29T15:55:59Z

On Sun, 2013-09-29 at 17:44 +0300, Gleb Natapov wrote:
> On Sun, Sep 29, 2013 at 07:52:28AM -0600, Alex Williamson wrote:
> > On Sun, 2013-09-29 at 16:16 +0300, Gleb Natapov wrote:
> > > On Thu, Sep 12, 2013 at 03:23:15PM -0600, Alex Williamson wrote:
> > > > So far we've succeeded at making KVM and VFIO mostly unaware of each
> > > > other, but there's any important point where that breaks down. Intel
> > > > VT-d hardware may or may not support snoop control. When snoop
> > > > control is available, intel-iommu promotes No-Snoop transactions on
> > > > PCIe to be cache coherent. That allows KVM to handle things like the
> > > > x86 WBINVD opcode as a nop. When the hardware does not support this,
> > > > KVM must implement a hardware visible WBINVD for the guest.
> > > >
> > > > We could simply let userspace tell KVM how to handle WBINVD, but it's
> > > > privileged for a reason. Allowing an arbitrary user to enable
> > > > physical WBINVD gives them a more access to the hardware. Previously,
> > > > this has only been enabled for guests supporting legacy PCI device
> > > > assignment. In such cases it's necessary for proper guest execution.
> > > > We therefore create a new KVM-VFIO virtual device. The user can add
> > > > and remove VFIO groups to this device via file descriptors. KVM
> > > > makes use of the VFIO external user interface to validate that the
> > > > user has access to physical hardware and gets the coherency state of
> > > > the IOMMU from VFIO. This provides equivalent functionality to
> > > > legacy KVM assignment, while keeping (nearly) all the bits isolated.
> > > >
> > > Looks good overall to me, one things though: to use legacy device
> > > assignment one needs root permission, so only root user can enable
> > > WBINVD emulation.
> >
> > That's not entirely accurate, legacy device assignment can be used by a
> > non-root user, libvirt does this all the time. The part that requires
> > root access is opening the pci-sysfs config file, the rest can be
> > managed via file permissions on the remaining sysfs files.
> >
> So how libvirt manages to do that as non-root user if pci-sysfs config
> file needs root permission. I didn't mean to say that legacy code
> checks for root explicitly, what I meant is that at some point root
> permission is needed.

Yes, libvirt needs admin permission for legacy to bind to pci-stub,
change permission on sysfs files and pass an opened pci config sysfs
file descriptor. For vfio libvirt needs admin permission to bind to
vfio-pci and change group file permission. From that perspective the
admin requirement is similar.

> > > Who does this permission checking here? Is only root
> > > allowed to create non coherent group with vfio?
> >
> > With vfio the user is granted permission by giving them access to the
> > vfio group file (/dev/vfio/$GROUP) and binding all the devices in the
> > group to vfio. That enables the user to create a container (~iommu
> > domain) with the group attached to it. Only then will the vfio external
> > user interface provide a reference to the group and enable this wbinvd
> > support. So, wbinvd emulation should only be available to a user that
> > "own" a vfio group and has it configured for use with this interface.
> What is the default permission of /dev/vfio/$GROUP?

It's 600. Thanks,

Alex