RE: [RFC] /dev/ioasid uAPI proposal

Tian, Kevin
2021-06-07T03:25:47Z

> From: Alex Williamson <alex.williamson@redhat.com>
> Sent: Saturday, June 5, 2021 5:29 AM
>
> On Fri, 4 Jun 2021 14:22:07 -0300
> Jason Gunthorpe <jgg@nvidia.com> wrote:
>
> > On Fri, Jun 04, 2021 at 06:10:51PM +0200, Paolo Bonzini wrote:
> > > On 04/06/21 18:03, Jason Gunthorpe wrote:
> > > > On Fri, Jun 04, 2021 at 05:57:19PM +0200, Paolo Bonzini wrote:
> > > > > I don't want a security proof myself; I want to trust VFIO to make the
> right
> > > > > judgment and I'm happy to defer to it (via the KVM-VFIO device).
> > > > >
> > > > > Given how KVM is just a device driver inside Linux, VMs should be a
> slightly
> > > > > more roundabout way to do stuff that is accessible to bare metal; not
> a way
> > > > > to gain extra privilege.
> > > >
> > > > Okay, fine, lets turn the question on its head then.
> > > >
> > > > VFIO should provide a IOCTL VFIO_EXECUTE_WBINVD so that userspace
> VFIO
> > > > application can make use of no-snoop optimizations. The ability of KVM
> > > > to execute wbinvd should be tied to the ability of that IOCTL to run
> > > > in a normal process context.
> > > >
> > > > So, under what conditions do we want to allow VFIO to giave a process
> > > > elevated access to the CPU:
> > >
> > > Ok, I would definitely not want to tie it *only* to CAP_SYS_RAWIO (i.e.
> > > #2+#3 would be worse than what we have today), but IIUC the proposal
> (was it
> > > yours or Kevin's?) was to keep #2 and add #1 with an enable/disable ioctl,
> > > which then would be on VFIO and not on KVM.
> >
> > At the end of the day we need an ioctl with two arguments:
> > - The 'security proof' FD (ie /dev/vfio/XX, or /dev/ioasid, or whatever)
> > - The KVM FD to control wbinvd support on
> >
> > Philosophically it doesn't matter too much which subsystem that ioctl
> > lives, but we have these obnoxious cross module dependencies to
> > consider..
> >
> > Framing the question, as you have, to be about the process, I think
> > explains why KVM doesn't really care what is decided, so long as the
> > process and the VM have equivalent rights.
> >
> > Alex, how about a more fleshed out suggestion:

Possibly just a naming thing, but I feel it's better to just talk about
no-snoop or non-coherent in the uAPI. Per Intel SDM wbinvd is a
privileged instruction. A process on the host has no privilege to
execute it. Only when this process holds a VM, this instruction matters
as there are guest privilege levels. But having VFIO uAPI (which is
userspace oriented) to explicitly deal with a CPU instruction which
makes sense only in a virtualization context sounds a bit weird...

> >
> > 1) When the device is attached to the IOASID via VFIO_ATTACH_IOASID
> > it communicates its no-snoop configuration:
>
> Communicates to whom?
>
> > - 0 enable, allow WBINVD
> > - 1 automatic disable, block WBINVD if the platform
> > IOMMU can police it (what we do today)
> > - 2 force disable, do not allow BINVD ever
>
> The only thing we know about the device is whether or not Enable
> No-snoop is hard wired to zero, ie. it either can't generate no-snoop
> TLPs ("coherent-only") or it might ("assumed non-coherent"). If
> we're putting the policy decision in the hands of userspace they should
> have access to wbinvd if they own a device that is assumed
> non-coherent AND it's attached to an IOMMU (page table) that is not
> blocking no-snoop (a "non-coherent IOASID").
>
> I think that means that the IOASID needs to be created (IOASID_ALLOC)
> with a flag that specifies whether this address space is coherent
> (IOASID_GET_INFO probably needs a flag/cap to expose if the system
> supports this). All mappings in this IOASID would use IOMMU_CACHE and

Yes, this sounds a cleaner way than specifying this attribute late in
VFIO_ATTACH_IOASID. Following Jason's proposal v2 will move to
the scheme requiring user to specify format info when creating an
IOASID. Leaving coherent out of that box just adds some trickiness,
e.g. whether allowing user to update page table between ALLOC
and ATTACH.

> and devices attached to it would be required to be backed by an IOMMU
> capable of IOMMU_CAP_CACHE_COHERENCY (attach fails otherwise). If only
> these IOASIDs exist, access to wbinvd would not be provided. (How does
> a user provided page table work? - reserved bit set, user error?)
>
> Conversely, a user could create a non-coherent IOASID and attach any
> device to it, regardless of IOMMU backing capabilities. Only if an
> assumed non-coherent device is attached would the wbinvd be allowed.
>
> I think that means that an EXECUTE_WBINVD ioctl lives on the IOASIDFD
> and the IOASID world needs to understand the device's ability to
> generate non-coherent DMA. This wbinvd ioctl would be a no-op (or
> some known errno) unless a non-coherent IOASID exists with a potentially
> non-coherent device attached.
>
> > vfio_pci may want to take this from an admin configuration knob
> > someplace. It allows the admin to customize if they want.
> >
> > If we can figure out a way to autodetect 2 from vfio_pci, all the
> > better
> >
> > 2) There is some IOMMU_EXECUTE_WBINVD IOCTL that allows userspace
> > to access wbinvd so it can make use of the no snoop optimization.
> >
> > wbinvd is allowed when:
> > - A device is joined with mode #0
> > - A device is joined with mode #1 and the IOMMU cannot block
> > no-snoop (today)
> >
> > 3) The IOASID's don't care about this at all. If IOMMU_EXECUTE_WBINVD
> > is blocked and userspace doesn't request to block no-snoop in the
> > IOASID then it is a userspace error.
>
> In my model above, the IOASID is central to this.
>
> > 4) The KVM interface is the very simple enable/disable WBINVD.
> > Possessing a FD that can do IOMMU_EXECUTE_WBINVD is required
> > to enable WBINVD at KVM.
>
> Right, and in the new world order, vfio is only a device driver, the
> IOASID manages the device's DMA. wbinvd is only necessary relative to
> non-coherent DMA, which seems like QEMU needs to bump KVM with an
> ioasidfd.
>
> > It is pretty simple from a /dev/ioasid perpsective, covers todays
> > compat requirement, gives some future option to allow the no-snoop
> > optimization, and gives a new option for qemu to totally block wbinvd
> > no matter what.
>
> What do you imagine is the use case for totally blocking wbinvd? In
> the model I describe, wbinvd would always be a no-op/known-errno when
> the IOASIDs are all allocated as coherent or a non-coherent IOASID has
> only coherent-only devices attached. Does userspace need a way to
> prevent itself from scenarios where wbvind is not a no-op?
>
> In general I'm having trouble wrapping my brain around the semantics of
> the enable/automatic/force-disable wbinvd specific proposal, sorry.
> Thanks,
>

Thanks,
Kevin