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

Paolo Bonzini
2022-09-28T15:08:38Z

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!

This is clearly a deficiency in the KVM kernel API, and (thanks to SRCU)
the kernel is the only place where you can have a *good* fix. It should
have been fixed years ago.

Paolo