Re: [PATCH 02/16] KVM: x86/mmu: Introduce a slot flag to zap only slot leafs on slot deletion

Huang, Kai
2024-05-15T23:36:18Z

On 16/05/2024 11:20 am, Sean Christopherson wrote:
> On Thu, May 16, 2024, Kai Huang wrote:
>>>> You had said up the thread, why not opt all non-normal VMs into the new
>>>> behavior. It will work great for TDX. But why do SEV and others want this
>>>> automatically?
>>>
>>> Because I want flexibility in KVM, i.e. I want to take the opportunity to try and
>>> break away from KVM's godawful ABI. It might be a pipe dream, as keying off the
>>> VM type obviously has similar risks to giving userspace a memslot flag. The one
>>> sliver of hope is that the VM types really are quite new (though less so for SEV
>>> and SEV-ES), whereas a memslot flag would be easily applied to existing VMs.
>>
>> Btw, does the "zap-leaf-only" approach always have better performance,
>> assuming we have to hold MMU write lock for that?
>
> I highly doubt it, especially given how much the TDP MMU can now do with mmu_lock
> held for read.
>
>> Consider a huge memslot being deleted/moved.
>>
>> If we can always have a better performance for "zap-leaf-only", then instead
>> of letting userspace to opt-in this feature, we perhaps can do the opposite:
>>
>> We always do the "zap-leaf-only" in KVM, but add a quirk for the VMs that
>> userspace know can have such bug and apply this quirk.
>
> Hmm, a quirk isn't a bad idea. It suffers the same problems as a memslot flag,
> i.e. who knows when it's safe to disable the quirk, but I would hope userspace
> would be much, much cautious about disabling a quirk that comes with a massive
> disclaimer.
>
> Though I suspect Paolo will shoot this down too ;-)

The quirk only works based on the assumption that userspace _exactly_
knows what kinda VMs will have this bug.

But as mentioned above, the first step is we need to convince ourselves
that doing "zap-leaf-only" by default is the right thing to do.

:-)