RE: [virtio-comment] [PATCH v1 1/8] admin: Add theory of operation for device migration

[Parav Pandit <parav@nvidia.com>]

> From: Jason Wang <jasowang@redhat.com>
> Sent: Tuesday, October 31, 2023 7:05 AM
> 
> On Mon, Oct 30, 2023 at 12:47âPM Parav Pandit <parav@nvidia.com> wrote:
> >
> >
> >
> > > From: virtio-comment@lists.oasis-open.org
> > > <virtio-comment@lists.oasis- open.org> On Behalf Of Jason Wang
> > >
> > > On Thu, Oct 26, 2023 at 11:45âAM Parav Pandit <parav@nvidia.com> wrote:
> > > >
> > > >
> > > >
> > > > > From: Jason Wang <jasowang@redhat.com>
> > > > > Sent: Thursday, October 26, 2023 6:16 AM
> > > > >
> > > > > On Wed, Oct 25, 2023 at 3:03âPM Parav Pandit <parav@nvidia.com>
> wrote:
> > > > > >
> > > > > >
> > > > > > > From: Jason Wang <jasowang@redhat.com>
> > > > > > > Sent: Wednesday, October 25, 2023 6:59 AM
> > > > > > > > For passthrough PASID assignment vq is not needed.
> > > > > > >
> > > > > > > How do you know that?
> > > > > > Because for passthrough, the hypervisor is not involved in
> > > > > > dealing with VQ at
> > > > > all.
> > > > >
> > > > > Ok, so if I understand correctly, you are saying your design
> > > > > can't work for the case of PASID assignment.
> > > > >
> > > > No. PASID assignment will happen from the guest for its own use
> > > > and device
> > > migration will just work fine because device context will capture this.
> > >
> > > It's not about device context. We're discussing "passthrough", no?
> > >
> > Not sure, we are discussing same.
> > A member device is passthrough to the guest, dealing with its own PASIDs and
> virtio interface for some VQ assignment to PASID.
> > So VQ context captured by the hypervisor, will have some PASID attached to
> this VQ.
> > Device context will be updated.
> >
> > > You want all virtio stuff to be "passthrough", but assigning a PASID
> > > to a specific virtqueue in the guest must be trapped.
> > >
> > No. PASID assignment to a specific virtqueue in the guest must go directly
> from guest to device.
> 
> This works like setting CR3, you can't simply let it go from guest to host.
> 
> Host IOMMU driver needs to know the PASID to program the IO page tables
> correctly.
>
This will be done by the IOMMU.
 
> > When guest iommu may need to communicate anything for this PASID, it will
> come through its proper IOMMU channel/hypercall.
> 
> Let's say using PASID X for queue 0, this knowledge is beyond the IOMMU scope
> but belongs to virtio. Or please explain how it can work when it goes directly
> from guest to device.
> 
We are yet to ever see spec for PASID to VQ assignment.
For ok for theory sake it is there.

Virtio driver will assign the PASID directly from guest driver to device using a create_vq(pasid=X) command.
Same process is somehow attached the PASID by the guest OS.
The whole PASID range is known to the hypervisor when the device is handed over to the guest VM.
So PASID mapping is setup by the hypervisor IOMMU at this point.

> > Virtio device is not the conduit for this exchange.
> >
> > > >
> > > > > >
> > > > > > > There are works ongoing to make vPASID work for the guest like
> vSVA.
> > > > > > > Virtio doesn't differ from other devices.
> > > > > > Passthrough do not run like SVA.
> > > > >
> > > > > Great, you find another limitation of "passthrough" by yourself.
> > > > >
> > > > No. it is not the limitation it is just the way it does not need
> > > > complex SVA to
> > > split the device for unrelated usage.
> > >
> > > How can you limit the user in the guest to not use vSVA?
> > >
> > He he, I am not limiting, again misunderstanding or wrong attribution.
> > I explained that hypervisor for passthrough does not need SVA.
> > Guest can do anything it wants from the guest OS with the member device.
> 
> Ok, so the point stills, see above.

I donât think so. The guest owns its PASID space and directly communicates like any other device attribute.

> 
> >
> > > >
> > > > > > Each passthrough device has PASID from its own space fully
> > > > > > managed by the
> > > > > guest.
> > > > > > Some cpu required vPASID and SIOV is not going this way anmore.
> > > > >
> > > > > Then how to migrate? Invent a full set of something else through
> > > > > another giant series like this to migrate to the SIOV thing?
> > > > > That's a mess for
> > > sure.
> > > > >
> > > > SIOV will for sure reuse most or all parts of this work, almost entirely as_is.
> > > > vPASID is cpu/platform specific things not part of the SIOV devices.
> > > >
> > > > > >
> > > > > > >
> > > > > > > > If at all it is done, it will be done from the guest by
> > > > > > > > the driver using virtio
> > > > > > > interface.
> > > > > > >
> > > > > > > Then you need to trap. Such things couldn't be passed
> > > > > > > through to guests
> > > > > directly.
> > > > > > >
> > > > > > Only PASID capability is trapped. PASID allocation and usage
> > > > > > is directly from
> > > > > guest.
> > > > >
> > > > > How can you achieve this? Assigning a PAISD to a device is
> > > > > completely
> > > > > device(virtio) specific. How can you use a general layer without
> > > > > the knowledge of virtio to trap that?
> > > > When one wants to map vPASID to pPASID a platform needs to be
> involved.
> > >
> > > I'm not talking about how to map vPASID to pPASID, it's out of the
> > > scope of virtio. I'm talking about assigning a vPASID to a specific
> > > virtqueue or other virtio function in the guest.
> > >
> > That can be done in the guest. The key is guest wont know that it is dealing
> with vPASID.
> > It will follow the same principle from your paper of equivalency, where virtio
> software layer will assign PASID to VQ and communicate to device.
> >
> > Anyway, all of this just digression from current series.
> 
> It's not, as you mention that only MSI-X is trapped, I give you another one.
> 
PASID access from the guest to be done fully by the guest IOMMU.
Not by virtio devices.

> >
> > > You need a virtio specific queue or capability to assign a PASID to
> > > a specific virtqueue, and that can't be done without trapping and
> > > without virito specific knowledge.
> > >
> > I disagree. PASID assignment to a virqueue in future from guest virtio driver to
> device is uniform method.
> > Whether its PF assigning PASID to VQ of self, Or VF driver in the
> > guest assigning PASID to VQ.
> >
> > All same.
> > Only IOMMU layer hypercalls will know how to deal with PASID assignment at
> platform layer to setup the domain etc table.
> >
> > And this is way beyond our device migration discussion.
> > By any means, if you were implying that somehow vq to PASID assignment
> _may_ need trap+emulation, hence whole device migration to depend on some
> trap+emulation, than surely, than I do not agree to it.
> 
> See above.
>
Yeah, I disagree to such implying.
 
> >
> > PASID equivalent in mlx5 world is ODP_MR+PD isolating the guest process and
> all of that just works on efficiency and equivalence principle already for a
> decade now without any trap+emulation.
> >
> > > > When virtio passthrough device is in guest, it has all its PASID accessible.
> > > >
> > > > All these is large deviation from current discussion of this
> > > > series, so I will keep
> > > it short.
> > > >
> > > > >
> > > > > > Regardless it is not relevant to passthrough mode as PASID is
> > > > > > yet another
> > > > > resource.
> > > > > > And for some cpu if it is trapped, it is generic layer, that
> > > > > > does not require virtio
> > > > > involvement.
> > > > > > So virtio interface asking to trap something because generic
> > > > > > facility has done
> > > > > in not the approach.
> > > > >
> > > > > This misses the point of PASID. How to use PASID is totally device
> specific.
> > > > Sure, and how to virtualize vPASID/pPASID is platform specific as
> > > > single PASID
> > > can be used by multiple devices and process.
> > >
> > > See above, I think we're talking about different things.
> > >
> > > >
> > > > >
> > > > > >
> > > > > > > > Capabilities of #2 is generic across all pci devices, so
> > > > > > > > it will be handled by the
> > > > > > > HV.
> > > > > > > > ATS/PRI cap is also generic manner handled by the HV and PCI
> device.
> > > > > > >
> > > > > > > No, ATS/PRI requires the cooperation from the vIOMMU. You
> > > > > > > can simply do ATS/PRI passthrough but with an emulated vIOMMU.
> > > > > > And that is not the reason for virtio device to build
> > > > > > trap+emulation for
> > > > > passthrough member devices.
> > > > >
> > > > > vIOMMU is emulated by hypervisor with a PRI queue,
> > > > PRI requests arrive on the PF for the VF.
> > >
> > > Shouldn't it arrive at platform IOMMU first? The path should be PRI
> > > -> RC -> IOMMU -> host -> Hypervisor -> vIOMMU PRI -> guest IOMMU.
> > >
> > Above sequence seems write.
> >
> > > And things will be more complicated when (v)PASID is used. So you
> > > can't simply let PRI go directly to the guest with the current architecture.
> > >
> > In current architecture of the pci VF, PRI does not go directly to the guest.
> > (and that is not reason to trap and emulate other things).
> 
> Ok, so beyond MSI-X we need to trap PRI, and we will probably trap other
> things in the future like PASID assignment.
PRI etc all belong to generic PCI 4K config space region.
Trap+emulation done in generic manner without involving virtio or other device types.

> 
> >
> > > >
> > > > > how can you pass
> > > > > through a hardware PRI request to a guest directly without trapping it
> then?
> > > > > What's more, PCIE allows the PRI to be done in a vendor (virtio)
> > > > > specific way, so you want to break this rule? Or you want to
> > > > > blacklist ATS/PRI
> > > for virtio?
> > > > >
> > > > I was aware of only pci-sig way of PRI.
> > > > Do you have a reference to the ECN that enables vendor specific
> > > > way of PRI? I
> > > would like to read it.
> > >
> > > I mean it doesn't forbid us to build a virtio specific interface for
> > > I/O page fault report and recovery.
> > >
> > So PRI of PCI does not allow. It is ODP kind of technique you meant above.
> > Yes one can build.
> > Ok. unrelated to device migration, so I will park this good discussion for later.
> 
> That's fine.
> 
> >
> > > > This will be very good to eliminate IOMMU PRI limitations.
> > >
> > > Probably.
> > >
> > > > PRI will directly go to the guest driver, and guest would interact
> > > > with IOMMU
> > > to service the paging request through IOMMU APIs.
> > >
> > > With PASID, it can't go directly.
> > >
> > When the request consist of PASID in it, it can.
> > But again these PCI-SIG extensions of PASID are not related to device
> migration, so I am differing it.
> >
> > > > For PRI in vendor specific way needs a separate discussion. It is
> > > > not related to
> > > live migration.
> > >
> > > PRI itself is not related. But the point is, you can't simply pass
> > > through ATS/PRI now.
> > >
> > Ah ok. the whole 4K PCI config space where ATS/PRI capabilities are located
> are trapped+emulated by hypervisor.
> > So?
> > So do we start emulating virito interfaces too for passthrough?
> > No.
> > Can one still continue to trap+emulate?
> > Sure why not?
> 
> Then let's not limit your proposal to be used by "passthrough" only?
One can possibly build some variant of the existing virtio member device using same owner and member scheme.
If for that is some admin commands are missing, may be one can add them.
No need to step on toes of use cases as they are different...

> I've shown you that
> 
> 1) you can't easily say you can pass through all the virtio facilities
> 2) how ambiguous for terminology like "passthrough"
>
It is not, it is well defined in v3, v2.
One can continue to argue and keep defining the variant and still call it data path acceleration and then claim it as passthrough ...
But I won't debate this anymore as its just non-technical aspects of least interest.
We have technical tasks and more improved specs to update going forward.
Working on extension for device specific contexts to enrich it.

> Thanks
> 
> >
> > Can one use AQ of this proposal to do so?
> > Sure, why not?