Re: KVM: x86: move vapic page handling out of fast path

[Marcelo Tosatti <mtosatti@redhat.com>]

On Sun, Jun 22, 2008 at 08:00:11AM +0300, Avi Kivity wrote:
> Marcelo Tosatti wrote:
>> I fail to see the point of handling the vapic page grab and ref
>> counting in __vcpu_run's heavyweight enter/exit path.
>>
>>   
>
> It's to avoid pinning the page indefinitely.
>
>> So move it to kvm_lapic_set_vapic_addr / kvm_free_lapic time.
>>
>> Other than the obvious improvement for non-Flexpriority case, this      
>> kills a down_read/up_read pair in heavy exits and reduces code size.    
>>   
>
> With mmu notifiers we can do this and still not ping the page:
>
> fast path:
>
>
> if (vapic_addr && !vapic_page)
>    enter_vapic();
>
>
> mmu notifier:
>
> if (gpa == vapic_addr)
>    exit_vapic()
>
>
> So let's wait with this until mmu notifiers are merged.

But what is the point, or advantage, of having the _any_ vapic page
handling in __vcpu_run ? 

The reference for it is grabbed at kvm_lapic_set_vapic_addr() (which
does not take any spinlock, so its safe to swapin the page) and released
at guest exit.

So, what do you have against this patch ?