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

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Message-ID: <6552edb5-f874-494a-08a4-381d0f438077@redhat.com>
Date:   Mon, 6 Aug 2018 13:44:49 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Xiao Guangrong <guangrong.xiao@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     rkrcmar@...hat.com, Vitaly Kuznetsov <vkuznets@...hat.com>,
        Junaid Shahid <junaids@...gle.com>,
        Xiao Guangrong <xiaoguangrong@...cent.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH] KVM: try __get_user_pages_fast even if not in atomic
 context

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