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

[Parav Pandit <parav@nvidia.com>]

> 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.
When guest iommu may need to communicate anything for this PASID, it will come through its proper IOMMU channel/hypercall.
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.

> >
> > > > 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.
 
> 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.

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).

> >
> > > 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.

> > 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?

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