Messages in this thread | | | Subject | Re: [PATCH v5 0/2] MTE support for KVM guest | From | Steven Price <> | Date | Mon, 7 Dec 2020 14:48:21 +0000 |
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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.
> I will update the status later next week!
Great, I look forward to hearing how it goes.
Thanks,
Steve
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