linux-kernel - Re: [PATCH 15/17] KVM: retpolines: x86: eliminate retpoline from vmx.c exit handlers

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Message-ID: <5e047696-c641-da4b-1215-ebecbb492417@redhat.com>
Date:   Wed, 25 Sep 2019 09:50:38 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Peter Xu <peterx@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 15/17] KVM: retpolines: x86: eliminate retpoline from
 vmx.c exit handlers

On 24/09/19 23:46, Andrea Arcangeli wrote:
>>>
>>> I would keep only EXIT_REASON_MSR_WRITE, EXIT_REASON_PREEMPTION_TIMER,
>>> EXIT_REASON_EPT_MISCONFIG and add EXIT_REASON_IO_INSTRUCTION.
>> Intuition doesn't work great when it comes to CPU speculative
>> execution runtime. I can however run additional benchmarks to verify
>> your theory that keeping around frequent retpolines will still perform
>> ok.
> On one most recent CPU model there's no measurable difference with
> your list or my list with a hrtimer workload (no cpuid). It's
> challenging to measure any difference below 0.5%.

Let's keep the short list then.

Paolo