Re: [PATCH 12/12] Accelerate nested SVM by emulating parts of GIF=0 v6

[Avi Kivity <avi@redhat.com>]

Alexander Graf wrote:
>>
>> Neat trick.  Have you looked at svm.c to see if we can cut out 
>> extraneous instructions (or move them out of the gif=0 area)?  Could 
>> mean big savings.
>
> Well for now I only emulate pre-vmrun and vmsave, because the emulator 
> breaks on lldt and set dr6 and ltr and ...
>

Yeah.  We could move some of these out, and emulate the rest (ltr I 
think is needed).

>>
>> Move to the regular x86 emulator, so if we extend it with debug flag 
>> support, privilege checking, etc, we get that for svm as well.
>
> Do you seriously want to have a callback to svm specific code in the 
> x86 emulator? 

That's what kvm_x86_ops is for.  We already emulate vendor specific 
instructions (vmcall and vmmcall).

> I was thinking of doing that, but this way looked way cleaner to me. I 
> would actually rather use prefixes as hint on pattern matching.
> So e.g. if we find a clgi instruction with some random prefix (or 
> somehow marked in other ways) we could just run a pattern that does 
> what the entry code would do. This way no emulation would be needed.

Smells like dynamic translation to me.  I don't see how you could do 
that generically -- it might work for one version of kvm, but not for 
other hypervisors.

>>> static int nested_svm_vmexit_real(struct vcpu_svm *svm, void *arg1,
>>>                   void *arg2, void *opaque)
>>> {
>>> @@ -1551,6 +1600,9 @@ static int nested_svm_vmexit(struct vcpu_svm 
>>> *svm)
>>>     kvm_mmu_reset_context(&svm->vcpu);
>>>     kvm_mmu_load(&svm->vcpu);
>>> +    /* KVM calls vmsave after vmrun, so let's run it now if we can */
>>> +    nested_svm_emulate(svm, NULL);
>>> +
>>>
>>
>> Will also call stgi eventually, so it may make sense to loop here too.
>
> See above - I did that, it broke, I tried to debug it for ~2 days and 
> just figured I'll leave it as is for now. 

Certainly, we can leave it for later.

> The current version gives _huge_ savings already.

Can you quantify?

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.