linux-kernel - Re: [PATCH] x86/split_lock: Avoid runtime reads of the TEST

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Message-ID: <CALCETrWmuTbHn9YCpGsWLBjR9rV1QEoEQ-m63NDd9cu7SecV6Q@mail.gmail.com>
Date:   Wed, 5 Feb 2020 17:18:23 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mark D Rustad <mrustad@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH] x86/split_lock: Avoid runtime reads of the TEST_CTRL MSR

On Wed, Feb 5, 2020 at 4:49 PM Luck, Tony <tony.luck@...el.com> wrote:
>
> In a context switch from a task that is detecting split locks
> to one that is not (or vice versa) we need to update the TEST_CTRL
> MSR. Currently this is done with the common sequence:
>         read the MSR
>         flip the bit
>         write the MSR
> in order to avoid changing the value of any reserved bits in the MSR.
>
> Cache the value of the TEST_CTRL MSR when we read it during initialization
> so we can avoid an expensive RDMSR instruction during context switch.

If something else that is per-cpu-ish gets added to the MSR in the
future, I will personally make fun of you for not making this percpu.