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

[Steven Price <steven.price@arm.com>]

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