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

Gleb Natapov
2013-09-30T13:23:02Z

On Sun, Sep 29, 2013 at 09:55:59AM -0600, Alex Williamson wrote:
> 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.
>
Yes, certainly appears so. Thanks.

--
Gleb.