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

Paolo Bonzini
2022-09-28T16:38:10Z

On 9/28/22 17:58, Sean Christopherson wrote:
> On Wed, Sep 28, 2022, Paolo Bonzini wrote:
>> On 9/27/22 17:58, Sean Christopherson wrote:
>>> 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
>
> Except that KVM currently does neither. If the fetch happens at CPL>0 and/or in
> L2, KVM injects #UD. That's flat out architecturally invalid. If it's a sticking
> point, the mmio.len==0 hack can be avoided by defining a new exit reason.

I agree that doing this at CPL>0 or in L2 is invalid and makes little
sense (because either way the invalid address cannot be reached without
help from the supervisor or L1's page tables).

>> 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.
>
> Userspace has the GPA and absolutely should be able to detect if the MMIO may have
> been due to its memslot manipulation versus the guest jumping into the weeds.
>
>> 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.
>
> I don't disagree that the memslots API is lacking, but IMO that is somewhat
> orthogonal to fixing KVM x86's "code fetch to MMIO" mess. Such a massive new API
> should be viewed and prioritized as a new feature, not as a bug fix, e.g. I'd
> like to have the luxury of being able to explore ideas beyond "let userspace
> batch memslot updates", and I really don't want to feel pressured to get this
> code reviewed and merge.

I absolutely agree that this is not a bugfix. Most new features for KVM
can be seen as bug fixes if you squint hard enough, but they're still
features.

> E.g. why do a batch update and not provide KVM_SET_ALL_USER_MEMORY_REGIONS to
> do wholesale replacement? That seems like it would be vastly simpler to handle
> on KVM's end. Or maybe there's a solution in the opposite direction, e.g. an
> API that allows 1->N or N->1 conversions but not arbitrary batching.

Wholesale replacement was my first idea when I looked at the issue, I
think at the end of 2020. I never got to a full implementation, but my
impression was that allocating/deallocating dirty bitmaps, rmaps etc.
would make it any easier than arbitrary batch updates.

> And just because QEMU's locking is "already pretty crappy", that's not a good
> reason to drag KVM down into the mud. E.g. taking a lock and conditionally
> releasing it... I get that this is an RFC, but IMO anything that requires such
> shenanigans simply isn't acceptable.
>
> /*
> * Takes kvm->slots_arch_lock, and releases it only if
> * invalid_slot allocation, kvm_prepare_memory_region failed
> * or batch->is_move_delete is true.
> */
> static int kvm_prepare_memslot(struct kvm *kvm,
> struct kvm_internal_memory_region_list *batch)
>

No objection about that. :)

Paolo