Re: [PATCH v5 0/2] MTE support for KVM guest

[Haibo Xu]

On Mon, 7 Dec 2020 at 22:48, Steven Price <steven.price@arm.com> wrote:
>
> On 04/12/2020 08:25, Haibo Xu wrote:
> > On Fri, 20 Nov 2020 at 17:51, Steven Price <steven.price@arm.com> wrote:
> >>
> >> On 19/11/2020 19:11, Marc Zyngier wrote:
> >>> On 2020-11-19 18:42, Andrew Jones wrote:
> >>>> On Thu, Nov 19, 2020 at 03:45:40PM +0000, Peter Maydell wrote:
> >>>>> On Thu, 19 Nov 2020 at 15:39, Steven Price <steven.price@arm.com> wrote:
> >>>>>> This series adds support for Arm's Memory Tagging Extension (MTE) to
> >>>>>> KVM, allowing KVM guests to make use of it. This builds on the
> >>>>> existing
> >>>>>> user space support already in v5.10-rc1, see [1] for an overview.
> >>>>>
> >>>>>> The change to require the VMM to map all guest memory PROT_MTE is
> >>>>>> significant as it means that the VMM has to deal with the MTE tags
> >>>>> even
> >>>>>> if it doesn't care about them (e.g. for virtual devices or if the VMM
> >>>>>> doesn't support migration). Also unfortunately because the VMM can
> >>>>>> change the memory layout at any time the check for PROT_MTE/VM_MTE has
> >>>>>> to be done very late (at the point of faulting pages into stage 2).
> >>>>>
> >>>>> I'm a bit dubious about requring the VMM to map the guest memory
> >>>>> PROT_MTE unless somebody's done at least a sketch of the design
> >>>>> for how this would work on the QEMU side. Currently QEMU just
> >>>>> assumes the guest memory is guest memory and it can access it
> >>>>> without special precautions...
> >>>>>
> >>>>
> >>>> There are two statements being made here:
> >>>>
> >>>> 1) Requiring the use of PROT_MTE when mapping guest memory may not fit
> >>>>     QEMU well.
> >>>>
> >>>> 2) New KVM features should be accompanied with supporting QEMU code in
> >>>>     order to prove that the APIs make sense.
> >>>>
> >>>> I strongly agree with (2). While kvmtool supports some quick testing, it
> >>>> doesn't support migration. We must test all new features with a migration
> >>>> supporting VMM.
> >>>>
> >>>> I'm not sure about (1). I don't feel like it should be a major problem,
> >>>> but (2).
> >>
> >> (1) seems to be contentious whichever way we go. Either PROT_MTE isn't
> >> required in which case it's easy to accidentally screw up migration, or
> >> it is required in which case it's difficult to handle normal guest
> >> memory from the VMM. I get the impression that probably I should go back
> >> to the previous approach - sorry for the distraction with this change.
> >>
> >> (2) isn't something I'm trying to skip, but I'm limited in what I can do
> >> myself so would appreciate help here. Haibo is looking into this.
> >>
> >
> > Hi Steven,
> >
> > Sorry for the later reply!
> >
> > I have finished the POC for the MTE migration support with the assumption
> > that all the memory is mapped with PROT_MTE. But I got stuck in the test
> > with a FVP setup. Previously, I successfully compiled a test case to verify
> > the basic function of MTE in a guest. But these days, the re-compiled test
> > can't be executed by the guest(very weird). The short plan to verify
> > the migration
> > is to set the MTE tags on one page in the guest, and try to dump the 
> > migrated
> > memory contents.
>
> Hi Haibo,
>
> Sounds like you are making good progress - thanks for the update. Have
> you thought about how the PROT_MTE mappings might work if QEMU itself
> were to use MTE? My worry is that we end up with MTE in a guest
> preventing QEMU from using MTE itself (because of the PROT_MTE
> mappings). I'm hoping QEMU can wrap its use of guest memory in a
> sequence which disables tag checking (something similar will be needed
> for the "protected VM" use case anyway), but this isn't something I've
> looked into.

As far as I can see, to map all the guest memory with PROT_MTE in VMM
is a little weird, and lots of APIs have to be changed to include this flag.
IMHO, it would be better if the KVM can provide new APIs to load/store the
guest memory tag which may make it easier to enable the Qemu migration
support.

>
> > I will update the status later next week!
>
> Great, I look forward to hearing how it goes.
>
> Thanks,
>
> Steve