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

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Date:   Tue, 3 Mar 2020 11:22:42 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     "Luck, Tony" <tony.luck@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        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>, Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH] x86/split_lock: Avoid runtime reads of the TEST_CTRL MSR

On Thu, Feb 06, 2020 at 11:37:04AM -0800, Andy Lutomirski wrote:
> 
> > On Feb 6, 2020, at 8:46 AM, Luck, Tony <tony.luck@...el.com> wrote:
> > 
> > On Wed, Feb 05, 2020 at 05:18:23PM -0800, Andy Lutomirski wrote:
> >>> 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.
> > 
> > Xiaoyao Li has posted a version using a percpu cache value:
> > 
> > https://lore.kernel.org/r/20200206070412.17400-4-xiaoyao.li@intel.com
> > 
> > So take that if it makes you happier.  My patch only used the
> > cached value to store the state of the reserved bits in the MSR
> > and assumed those are the same for all cores.
> > 
> > Xiaoyao Li's version updates with what was most recently written
> > on each thread (but doesn't, and can't, make use of that because we
> > know that the other thread on the core may have changed the actual
> > value in the MSR).
> > 
> > If more bits are implemented that need to be set at run time, we
> > are likely up the proverbial creek. I'll see if I can find out if
> > there are plans for that.
> > 
> 
> I suppose that this whole thing is a giant mess, especially since at least
> one bit there is per-physical-core. Sigh.
> 
> So I don’t have a strong preference.

I'd prefer to go with this patch, i.e. not percpu, to remove the temptation
of incorrectly optimizing away toggling SPLIT_LOCK_DETECT.