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

[Jason Wang <jasowang@redhat.com>]

On Wed, Nov 22, 2023 at 12:27âAM Parav Pandit <parav@nvidia.com> wrote:
>
>
> > From: Jason Wang <jasowang@redhat.com>
> > Sent: Tuesday, November 21, 2023 10:15 AM
> >
> > On Fri, Nov 17, 2023 at 11:09âPM Michael S. Tsirkin <mst@redhat.com>
> > wrote:
> > >
> > > On Fri, Nov 17, 2023 at 02:51:04PM +0000, Parav Pandit wrote:
> > > >
> > > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > > Sent: Friday, November 17, 2023 7:24 PM
> > > > >
> > > > > On Fri, Nov 17, 2023 at 12:46:21PM +0000, Parav Pandit wrote:
> > > > > >
> > > > > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > > > > Sent: Friday, November 17, 2023 6:00 PM
> > > > > > > To: Parav Pandit <parav@nvidia.com>
> > > > > > >
> > > > > > > On Fri, Nov 17, 2023 at 12:02:49PM +0000, Parav Pandit wrote:
> > > > > > > >
> > > > > > > >
> > > > > > > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > > > > > > Sent: Friday, November 17, 2023 5:13 PM
> > > > > > > > >
> > > > > > > > > On Fri, Nov 17, 2023 at 11:20:14AM +0000, Parav Pandit wrote:
> > > > > > > > > >
> > > > > > > > > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > > > > > > > > Sent: Friday, November 17, 2023 4:41 PM
> > > > > > > > > > >
> > > > > > > > > > > On Fri, Nov 17, 2023 at 10:20:45AM +0000, Parav Pandit
> > wrote:
> > > > > > > > > > > >
> > > > > > > > > > > >
> > > > > > > > > > > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > > > > > > > > > > Sent: Friday, November 17, 2023 3:38 PM
> > > > > > > > > > > > >
> > > > > > > > > > > > > On Wed, Nov 15, 2023 at 05:39:43PM +0000, Parav Pandit
> > wrote:
> > > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > > Additionally, if hypervisor has put the trap
> > > > > > > > > > > > > > > > on virtio config, and because the memory
> > > > > > > > > > > > > > > > device already has the interface for virtio
> > > > > > > > > > > > > > > > config,
> > > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > > Hypervisor can directly write/read from the
> > > > > > > > > > > > > > > > virtual config to the member's
> > > > > > > > > > > > > > > config space, without going through the device context,
> > right?
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > If it can do it or it can choose to not. I
> > > > > > > > > > > > > > > don't see how it is related to the discussion here.
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > It is. I donât see a point of hypervisor not
> > > > > > > > > > > > > > using the native interface provided
> > > > > > > > > > > > > by the member device.
> > > > > > > > > > > > >
> > > > > > > > > > > > > So for example, it seems reasonable to a member
> > > > > > > > > > > > > supporting both existing pci register interface
> > > > > > > > > > > > > for compatibility and the future DMA based one for
> > > > > > > > > > > > > scale. In such a case, it seems possible that DMA
> > > > > > > > > > > > > will expose more features than pci. And then a
> > > > > > > > > > > > > hypervisor might decide to use
> > > > > > > > > > > that in preference to pci registers.
> > > > > > > > > > > >
> > > > > > > > > > > > We donât find it right to involve owner device for
> > > > > > > > > > > > mediating at current scale
> > > > > > > > > > >
> > > > > > > > > > > In this model, device will be its own owner. Should not be a
> > problem.
> > > > > > > > > > >
> > > > > > > > > > I didnât understand above comment.
> > > > > > > > >
> > > > > > > > > We'd add a new group type "self". You can then send admin
> > > > > > > > > commands through VF itself not through PF.
> > > > > > > > >
> > > > > > > > How? The device is owned by the guest. FLR and device reset
> > > > > > > > cannot send the
> > > > > > > admin command reliably.
> > > > > > >
> > > > > > > It's of the "it hurts when I do this - don't do this then" category.
> > > > > > >
> > > > > > it is donât do medication category, yes due all this weirdness
> > > > > > that has been
> > > > > asked.
> > > > > >
> > > > > > >
> > > > > > > > >
> > > > > > > > > > > > and to not break TDISP efforts in upcoming time by such
> > design.
> > > > > > > > > > >
> > > > > > > > > > > Look you either stop mentioning TDISP as motivation or
> > > > > > > > > > > actually try to address it. Safe migration with TDISP is really
> > hard.
> > > > > > > > > > But that is not an excuse to say that TDISP migration is
> > > > > > > > > > not present, hence
> > > > > > > > > involve the owner device for config space access.
> > > > > > > > > > This is another hurdle added that further blocks us away from
> > TDISP.
> > > > > > > > > > Hence, we donât want to take the route of involving
> > > > > > > > > > owner device for any
> > > > > > > > > config access.
> > > > > > > > >
> > > > > > > > > This "blocks" is all just wild hunches. hypervisor
> > > > > > > > > controls some aspects of TDISP devices for sure - maybe we
> > > > > > > > > actually should use pci config space as that is generally hypervisor
> > controlled.
> > > > > > > > Even bad to do hypercalls.
> > > > > > > > I showed you last time the role of the PCI config space
> > > > > > > > snippet from the
> > > > > spec.
> > > > > > >
> > > > > > > Yes I remember. This is just an example though. My point is
> > > > > > > maybe it is solvable maybe it is not.
> > > > > > >
> > > > > > > > Do you see we are repeating the discussion again?
> > > > > > >
> > > > > > > One of the reasons is that people bring up irrelevances. TDISP
> > > > > > > is important but has to be addressed or deferred not vaguely referred
> > to.
> > > > > >
> > > > > > So lets continue to follow the current TDISP direction of not
> > > > > > involving
> > > > > hypervisor for virtio common and device config.
> > > >
> > > > If you disagree to it, please speak now, so that we donât debate on this
> > again in next 3 days.
> > > > Because this is the fundamental design considerations it relied on.
> > > > There is no point going forward if you want to disagree to it.
> > > > Other variants are fine, but other variants cannot be the only choice.
> > > >
> > > > > >
> > > > > > >
> > > > > > > > >
> > > > > > > > > > > For example, your current patches are clearly broken for TDISP:
> > > > > > > > > > > owner can control queue state at any time making
> > > > > > > > > > > device modify memory in any way it wants.
> > > > > > > > > > >
> > > > > > > > > > When TDISP migration is needed, the admin device can be
> > > > > > > > > > another TVM
> > > > > > > > > outside the HV scope.
> > > > > > > > > > Or an alternative would have device context encrypted
> > > > > > > > > > not visible to HV at
> > > > > > > all.
> > > > > > > > >
> > > > > > > > > Maybe. Fact remains your patches do conflict with TDISP
> > > > > > > > > and you seem to be fine with it because you have a hunch you can
> > fix it.
> > > > > > > > > But we can't do development based on your hunches.
> > > > > > > > >
> > > > > > > > We have different view.
> > > > > > > > My patches do not conflict with TDISP because TDISP has
> > > > > > > > clear definition of
> > > > > > > not involving hypervisor for transport.
> > > > > > > > And that part is still preserved.
> > > > > > > > Delegating the migration to another TDISP or encrypting is
> > > > > > > > yet to be
> > > > > defined.
> > > > > > > > And current patches will align to both the approaches in future.
> > > > > > > >
> > > > > > > > So you need to re-evaluate your judgment.
> > > > > > >
> > > > > > > If you like they do not "conflict".  But if used with TDISP
> > > > > > > they just make it insecure and thus completely worthless.  If
> > > > > > > hypervisor can change ring state to make device poke at random
> > > > > > > guest memory then it is game over and all the effort spent was
> > security theater.
> > > > > > Not really, I proposed two options.
> > > > > > 1. delegate the task of LM to the TVM. (proposed by two cpu vendors).
> > > > > > In this case all the infra we build here, just works fine.
> > > > >
> > > > > I think modification will be needed: currently commands are sent
> > > > > through the PF, and that is under hypervisor control.
> > > > > You should not assign PF to TVM.
> >
> > That's the point. And that's why it keeps people confused to believe the
> > current PF/adminq can work in the TDISP.
> >
> There is no confusion.

No, when LingShan points out the conflict, you just told us it will be
addressed in the future.

And after Michael pointed it out again, you agree than adminq can not
be part of PF in this context.

And you miss the fact that admin virtqueue today can't be used to
manage the owner.

> The admin queue interface ensures first step that TDISP interface is dedicated to guest as today.
> There is no bifurcation added on the VF that needs extra mediation.
>
> > > > Yes, an admin virtio function will be there which will do the admin
> > commands listed.
> > >
> > > So it can't be PF, so at least we need a new group type.
> > > I am inclined to then say, operate it through VF itself.
> >
> > So it exactly matches the idea of transport virtqueue (a per VF/SF one).
> >
> There is no need for transport virtqueue for VF as VF device has same uniform principle as PF.

I don't understand here. I have explained that you have invented a
function duplication of transport virtqueue.

> If you want transport vq, please have it on the PF too.

Nothing prevents this, actually transport virtqueue start from this.

> And that also is not needed because there is already CVQ.

I don't see why you keep mentioning CVQ. I don't see anyone that says
transport virtqueue is going to replace CVQ.

>
> > But it still requires a PCI part to bootstrap.
> >
> > >
> > >
> > > > >
> > > > > > It also does not require any hypervisor mediation for control plane.
> > > > > >
> > > > > > 2. Encrypt the owner device workload to be not seen by
> > > > > > hypervisor
> > > > > >
> > > > > > Both methods does not affect the current direction.
> > > > > >
> > > > > > But if we force trap+emulation, it is 100% broken for TDISP.
> > > > > > And I would not promote that.
> > > > > >
> > > > > > > But you know this, don't you? This is why you mentioned encrypting
> > device.
> > > > > > > Maybe that works. It just does not work *as is*.
> > > > > > It works as_is. But current infrastructure does not block the future
> > work.
> > > > > >
> > > > > > >
> > > > > > >
> > > > > > > > >
> > > > > > > > > > Such encryption is not possible, with the trap+emulation
> > > > > > > > > > method, where HV
> > > > > > > > > will have to decrypt the data coming over MMIO writes.
> > > > > > > > >
> > > > > > > > > I don't how what trap+emulation has to do with it. Do you
> > > > > > > > > refer to the shadow vq thing?
> > > > > > > >
> > > > > > > > The method proposed here does not hinder any TDISP direction.
> > > > > > >
> > > > > > > direction? No, why would it. we can always add more commands
> > > > > > > that are safe for TDISP. commands you propose here are unsafe for
> > TDISP.
> > > > > > >
> > > > > > > > Without my proposal, do you have a method that does not
> > > > > > > > involve hypervisor
> > > > > > > intervention for virtio common and device config space, cvq
> > > > > > > and shadow
> > > > > vq?
> > > > > > > > If so, I would like to hear that as well because that will align with
> > TDISP.
> > > > > > >
> > > > > > > I really did not give it much thought.  I suspect for TDISP it
> > > > > > > just might be cleaner to have guest agent migrate device.
> > > > > > > Certainly removes all
> > > > > the messy questions.
> > > > > > > That, to me impliest there needs to be a way to send migration
> > > > > > > commands through VF itself. Does this "involve hypervisor
> > > > > > > intervention"? No one should care I think.
> > > > > > Too far of the future to envision. May be yes. When such
> > > > > > platform is built, for sure whoever migrates need migrate its device side
> > too.
> > > > > > Some knowledge of migration driver is needed.
> > > > >
> > > > > So TDISP migration is so far in the future you do not need to bother
> > about it.
> > > > > Fine. Then don't bring it up pls.
> > > > >
> > > > As long as we are aligned to the requirement that a virtio member device is
> > mapped to the guest VM without mediating the virtio interface, I am good.
> > > > Again, other variants are fine, but above listed mapped variant is the
> > minimum variant needed.
> > >
> > > I think it's worth supporting this. I wouldn't call this minimum there
> > > are other approaches.  And I am not so sure it's worth trying to
> > > support this in all kind of systems such as IOMMU without dirty bit
> > > support. If some old systems will need mediation, this is kind of like
> > > legacy interface. Not a big deal.
> > >
> >
> > +1
>
> There are users with the recent cpus that may not have the IOMMU dirty page tracking support.
> So I donât fully agree.

There are setups that don't have SR-IOV or even PCI.

Let's have a unified standard.

Thanks

>