RE: [PATCH v1 2/2] vfio/pci: Emulate PASID/PRI capability for VFs

[Tian, Kevin]

> From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Sent: Tuesday, April 14, 2020 11:24 PM
> 
> On Tue, 14 Apr 2020 03:42:42 +0000
> "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
> 
> > > From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > > Sent: Tuesday, April 14, 2020 11:29 AM
> > >
> > > On Tue, 14 Apr 2020 02:40:58 +0000
> > > "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
> > >
> > > > > From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > > > > Sent: Tuesday, April 14, 2020 3:21 AM
> > > > >
> > > > > On Mon, 13 Apr 2020 08:05:33 +0000
> > > > > "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
> > > > >
> > > > > > > From: Tian, Kevin
> > > > > > > Sent: Monday, April 13, 2020 3:55 PM
> > > > > > >
> > > > > > > > From: Raj, Ashok <ashok.raj@xxxxxxxxxxxxxxx>
> > > > > > > > Sent: Monday, April 13, 2020 11:11 AM
> > > > > > > >
> > > > > > > > On Wed, Apr 08, 2020 at 10:19:40AM -0600, Alex Williamson
> wrote:
> > > > > > > > > On Tue, 7 Apr 2020 21:00:21 -0700
> > > > > > > > > "Raj, Ashok" <ashok.raj@xxxxxxxxx> wrote:
> > > > > > > > >
> > > > > > > > > > Hi Alex
> > > > > > > > > >
> > > > > > > > > > + Bjorn
> > > > > > > > >
> > > > > > > > >  + Don
> > > > > > > > >
> > > > > > > > > > FWIW I can't understand why PCI SIG went different ways
> with
> > > ATS,
> > > > > > > > > > where its enumerated on PF and VF. But for PASID and PRI its
> > > only
> > > > > > > > > > in PF.
> > > > > > > > > >
> > > > > > > > > > I'm checking with our internal SIG reps to followup on that.
> > > > > > > > > >
> > > > > > > > > > On Tue, Apr 07, 2020 at 09:58:01AM -0600, Alex Williamson
> > > wrote:
> > > > > > > > > > > > Is there vendor guarantee that hidden registers will locate
> at
> > > the
> > > > > > > > > > > > same offset between PF and VF config space?
> > > > > > > > > > >
> > > > > > > > > > > I'm not sure if the spec really precludes hidden registers,
> but
> > > the
> > > > > > > > > > > fact that these registers are explicitly outside of the
> capability
> > > > > > > > > > > chain implies they're only intended for device specific use,
> so
> > > I'd
> > > > > say
> > > > > > > > > > > there are no guarantees about anything related to these
> > > registers.
> > > > > > > > > >
> > > > > > > > > > As you had suggested in the other thread, we could consider
> > > > > > > > > > using the same offset as in PF, but even that's a better guess
> > > > > > > > > > still not reliable.
> > > > > > > > > >
> > > > > > > > > > The other option is to maybe extend driver ops in the PF to
> > > expose
> > > > > > > > > > where the offsets should be. Sort of adding the quirk in the
> > > > > > > > > > implementation.
> > > > > > > > > >
> > > > > > > > > > I'm not sure how prevalent are PASID and PRI in VF devices. If
> > > SIG is
> > > > > > > > resisting
> > > > > > > > > > making VF's first class citizen, we might ask them to add
> some
> > > > > verbiage
> > > > > > > > > > to suggest leave the same offsets as PF open to help
> emulation
> > > > > software.
> > > > > > > > >
> > > > > > > > > Even if we know where to expose these capabilities on the VF,
> it's
> > > not
> > > > > > > > > clear to me how we can actually virtualize the capability itself.
> If
> > > > > > > > > the spec defines, for example, an enable bit as r/w then
> software
> > > that
> > > > > > > > > interacts with that register expects the bit is settable.  There's
> no
> > > > > > > > > protocol for "try to set the bit and re-read it to see if the
> hardware
> > > > > > > > > accepted it".  Therefore a capability with a fixed enable bit
> > > > > > > > > representing the state of the PF, not settable by the VF, is
> > > > > > > > > disingenuous to the spec.
> > > > > > > >
> > > > > > > > I think we are all in violent agreement. A lot of times the pci spec
> > > gets
> > > > > > > > defined several years ahead of real products and no one
> > > remembers
> > > > > > > > the justification on why they restricted things the way they did.
> > > > > > > >
> > > > > > > > Maybe someone early product wasn't quite exposing these
> features
> > > to
> > > > > the
> > > > > > > > VF
> > > > > > > > and hence the spec is bug compatible :-)
> > > > > > > >
> > > > > > > > >
> > > > > > > > > If what we're trying to do is expose that PASID and PRI are
> enabled
> > > on
> > > > > > > > > the PF to a VF driver, maybe duplicating the PF capabilities on
> the
> > > VF
> > > > > > > > > without the ability to control it is not the right approach.
> Maybe
> > > we
> > > > > > > >
> > > > > > > > As long as the capability enable is only provided when the PF has
> > > > > enabled
> > > > > > > > the feature. Then it seems the hardware seems to do the right
> thing.
> > > > > > > >
> > > > > > > > Assume we expose PASID/PRI only when PF has enabled it. It will
> be
> > > the
> > > > > > > > case since the PF driver needs to exist, and IOMMU would have
> set
> > > the
> > > > > > > > PASID/PRI/ATS on PF.
> > > > > > > >
> > > > > > > > If the emulation is purely spoofing the capability. Once vIOMMU
> > > driver
> > > > > > > > enables PASID, the context entries for the VF are completely
> > > > > independent
> > > > > > > > from the PF context entries.
> > > > > > > >
> > > > > > > > vIOMMU would enable PASID, and we just spoof the PASID
> > > capability.
> > > > > > > >
> > > > > > > > If vIOMMU or guest for some reason does disable_pasid(), then
> the
> > > > > > > > vIOMMU driver can disaable PASID on the VF context entries. So
> the
> > > VF
> > > > > > > > although the capability is blanket enabled on PF, IOMMU
> gaurantees
> > > > > the
> > > > > > > > transactions are blocked.
> > > > > > > >
> > > > > > > >
> > > > > > > > In the interim, it seems like the intent of the virtual capability
> > > > > > > > can be honored via help from the IOMMU for the controlling
> aspect..
> > > > > > > >
> > > > > > > > Did i miss anything?
> > > > > > >
> > > > > > > Above works for emulating the enable bit (under the assumption
> that
> > > > > > > PF driver won't disable pasid when vf is assigned). However, there
> are
> > > > > > > also "Execute permission enable" and "Privileged mode enable"
> bits in
> > > > > > > PASID control registers. I don't know how those bits could be
> cleanly
> > > > > > > emulated when the guest writes a value different from PF's...
> > > > > >
> > > > > > sent too quick. the IOMMU also includes control bits for allowing/
> > > > > > blocking execute requests and supervisor requests. We can rely on
> > > > > > IOMMU to block those requests to emulate the disabled cases of
> > > > > > all three control bits in the pasid cap.
> > > > >
> > > > >
> > > > > So if the emulation of the PASID capability takes into account the
> > > > > IOMMU configuration to back that emulation, shouldn't we do that
> > > > > emulation in the hypervisor, ie. QEMU, rather than the kernel vfio
> > > > > layer?  Thanks,
> > > > >
> > > > > Alex
> > > >
> > > > We need enforce it in physical IOMMU, to ensure that even the
> > > > VF may send requests which violate the guest expectation those
> > > > requests are always blocked by IOMMU. Kernel vfio identifies
> > > > such need when emulating the pasid cap and then forward the
> > > > request to host iommu driver.
> > >
> > > Implementing this in the kernel would be necessary if we needed to
> > > protect from the guest device doing something bad to the host or
> > > other devices.  Making sure the physical IOMMU is configured to meet
> > > guest expectations doesn't sound like it necessarily falls into that
> > > category.  We do that on a regular basis to program the DMA mappings.
> > > Tell me more about why the hypervisor can't handle this piece of
> > > guest/host synchronization on top of all the other things it
> > > synchronizes to make a VM.  Thanks,
> > >
> >
> > I care more about "execution permission" and "privileged mode".
> > It might be dangerous when the guest disallows the VF from sending
> 
> "Dangerous" how?  We're generally ok with the user managing their own
> consistency, it's when the user can affect other users/devices that we
> require vfio in the kernel to actively manage something.  There's a very
> different scope to the vfio-pci kernel module implementing a fake
> capability and trying to make it behave indistinguishably from the real
> capability versus a userspace driver piecing together an emulation
> that's good enough for their purposes.  Thanks,
>

How could emulation fix this gap when the VF DMAs don't go through
the vIOMMU? What you explained all makes sense before talking about
the emulation of PASID capability, i.e. vfio only cares about isolation 
between assigned devices. However now vfio exposes a capability 
which is shared by PF/VF while pure software emulation may break 
the guest expectation, and now the only viable mitigation is to get 
the help from physical IOMMU. then why cannot vfio include such 
mitigation in its emulation of the PASID capability? 

Thanks
Kevin

> 
> > DMA requests which have the execute or privileged bit set while the VF
> > can still do so w/o proper protection in the IOMMU. This is all about
> > vSVA for vfio devices, where the guest page table is directly linked in
> > IOMMU and vfio doesn't participate in the specific DMA mappings. if
> > an emulated device includes a pasid cap, Qemu vIOMMU will handle
> > it for sure.
> >
> > Thanks
> > Kevin
> >