Re: [RFC PATCH 0/9] kvm: implement atomic memslot updates

David Hildenbrand
2022-09-28T15:34:08Z

On 28.09.22 17:07, Paolo Bonzini wrote:
> On 9/27/22 17:58, Sean Christopherson wrote:
>> On Tue, Sep 27, 2022, Emanuele Giuseppe Esposito wrote:
>>>
>>> Am 26/09/2022 um 23:28 schrieb Sean Christopherson:
>>>> On Mon, Sep 26, 2022, David Hildenbrand wrote:
>>>>> As Sean said "This is an awful lot of a complexity to take on for something
>>>>> that appears to be solvable in userspace."
>>>>
>>>> And if the userspace solution is unpalatable for whatever reason, I'd like to
>>>> understand exactly what KVM behavior is problematic for userspace. E.g. the
>>>> above RHBZ bug should no longer be an issue as the buggy commit has since been
>>>> reverted.
>>>
>>> It still is because I can reproduce the bug, as also pointed out in
>>> multiple comments below.
>>
>> You can reproduce _a_ bug, but it's obviously not the original bug, because the
>> last comment says:
>>
>> Second, indeed the patch was reverted and somehow accepted without generating
>> too much noise:
>>
>> ...
>>
>> The underlying issue of course as we both know is still there.
>>
>> You might have luck reproducing it with this bug
>>
>> https://bugzilla.redhat.com/show_bug.cgi?id=1855298
>>
>> But for me it looks like it is 'working' as well, so you might have
>> to write a unit test to trigger the issue.
>>
>>>> If the issue is KVM doing something nonsensical on a code fetch to MMIO, then I'd
>>>> much rather fix _that_ bug and improve KVM's user exit ABI to let userspace handle
>>>> the race _if_ userspace chooses not to pause vCPUs.
>>>>
>>>
>>> Also on the BZ they all seem (Paolo included) to agree that the issue is
>>> non-atomic memslots update.
>>
>> Yes, non-atomic memslot likely results in the guest fetching from a GPA without a
>> memslot. I'm asking for an explanation of exactly what happens when that occurs,
>> because it should be possible to adjust KVM and/or QEMU to play nice with the
>> fetch, e.g. to resume the guest until the new memslot is installed, in which case
>> an atomic update isn't needed.
>>
>> I assume the issue is that KVM exits with KVM_EXIT_INTERNAL_ERROR because the
>> guest is running at CPL=0, and QEMU kills the guest in response. If that's correct,
>> then that problem can be solved by exiting to userspace with KVM_EXIT_MMIO instead
>> of KVM_EXIT_INTERNAL_ERROR so that userspace can do something sane in response to
>> the MMIO code fetch.
>>
>> I'm pretty sure this patch will Just Work for QEMU, because QEMU simply resumes
>> the vCPU if mmio.len==0. It's a bit of a hack, but I don't think it violates KVM's
>> ABI in any way, and it can even become "official" behavior since KVM x86 doesn't
>> otherwise exit with mmio.len==0.
>
> I think this patch is not a good idea for two reasons:
>
> 1) we don't know how userspace behaves if mmio.len is zero. It is of
> course reasonable to do nothing, but an assertion failure is also a
> valid behavior
>
> 2) more important, there is no way to distinguish a failure due to the
> guest going in the weeds (and then KVM_EXIT_INTERNAL_ERROR is fine) from
> one due to the KVM_SET_USER_MEMORY_REGION race condition. So this will
> cause a guest that correctly caused an internal error to loop forever.
>
> While the former could be handled in a "wait and see" manner, the latter
> in particular is part of the KVM_RUN contract. Of course it is possible
> for a guest to just loop forever, but in general all of KVM, QEMU and
> upper userspace layers want a crashed guest to be detected and stopped
> forever.
>
> Yes, QEMU could loop only if memslot updates are in progress, but
> honestly all the alternatives I have seen to atomic memslot updates are
> really *awful*. David's patches even invent a new kind of mutex for
> which I have absolutely no idea what kind of deadlocks one should worry
> about and why they should not exist; QEMU's locking is already pretty
> crappy, it's certainly not on my wishlist to make it worse!

Just to comment on that (I'm happy as long as this gets fixed), a simple
mutex with trylock should get the thing done as well -- kicking the VCPU
if the trylock fails. But I did not look further into locking alternatives.

--
Thanks,

David / dhildenb