Re: [PATCH] KVM: try __get_user_pages_fast even if not in atomic context

[Paolo Bonzini]

On 06/08/2018 09:51, Xiao Guangrong wrote:
> 
> 
> On 07/27/2018 11:46 PM, Paolo Bonzini wrote:
>> We are currently cutting hva_to_pfn_fast short if we do not want an
>> immediate exit, which is represented by !async && !atomic. However,
>> this is unnecessary, and __get_user_pages_fast is *much* faster
>> because the regular get_user_pages takes pmd_lock/pte_lock.
>> In fact, when many CPUs take a nested vmexit at the same time
>> the contention on those locks is visible, and this patch removes
>> about 25% (compared to 4.18) from vmexit.flat on a 16 vCPU
>> nested guest.
>>
> 
> Nice improvement.
> 
> Then after that, we will unconditionally try hva_to_pfn_fast(), does
> it hurt the case that the mappings in the host's page tables have not
> been present yet?

I don't think so, because that's quite slow anyway.

> Can we apply this tech to other places using gup or even squash it
> into get_user_pages()?

That may make sense.  Andrea, do you have an idea?

Paolo