linux-kernel - Re: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on Hyper-V

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Message-ID: <87r2opcr6u.fsf@vitty.brq.redhat.com>
Date:   Mon, 12 Mar 2018 15:19:05 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Cathy Avery <cavery@...hat.com>, Bandan Das <bsd@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on Hyper-V

Thomas Gleixner <tglx@...utronix.de> writes:

> On Fri, 9 Mar 2018, Vitaly Kuznetsov wrote:
>
>> Enlightened VMCS is just a structure in memory, the main benefit
>> besides avoiding somewhat slower VMREAD/VMWRITE is using clean field
>> mask: we tell the underlying hypervisor which fields were modified
>> since VMEXIT so there's no need to inspect them all.
>> 
>> Tight CPUID loop test shows significant speedup:
>> Before: 18890 cycles
>> After: 8304 cycles
>> 
>> Static key is being used to avoid performance penalty for non-Hyper-V
>> deployments. Tests show we add around 3 (three) CPU cycles on each
>> VMEXIT (1077.5 cycles before, 1080.7 cycles after for the same CPUID
>> loop on bare metal). We can probably avoid one test/jmp in vmx_vcpu_run()
>> but I don't see a clean way to use static key in assembly.
>
> STATIC_JUMP_IF_TRUE, STATIC_JUMP_IF_FALSE are your friends.
>

Thanks for the tip,

with a single kernel user of these APIs it was easy to miss :-)

Unfortunately, these APIs are only present if HAVE_JUMP_LABEL and
(afaiu) we still care about KVM on !HAVE_JUMP_LABEL builds. It would be
nice if we can make them behave the same way static_branch_likely() and
friends do: compile into something else when !HAVE_JUMP_LABEL so we can
avoid nasty #ifdefs in C code.

That said I'd like to defer the question to KVM maintainers: Paolo,
Radim, what would you like me to do? Use STATIC_JUMP_IF_TRUE/FALSE as
they are, try to make them work for !HAVE_JUMP_LABEL and use them or
maybe we can commit the series as-is and have it as a future
optimization (e.g. when HAVE_JUMP_LABEL becomes mandatory)?

-- 
  Vitaly