linux-kernel - Re: [PATCH v16] x86/split_lock: Enable split lock detection by kernel

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Message-ID: <20200126200124.GA30377@agluck-desk2.amr.corp.intel.com>
Date:   Sun, 26 Jan 2020 12:01:24 -0800
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        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>, Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH v16] x86/split_lock: Enable split lock detection by kernel

On Sat, Jan 25, 2020 at 04:34:29PM -0800, Andy Lutomirski wrote:
> Although I suppose the pile of wrmsrl_safes() in the existing patch
> might be sufficient.
> 
> All this being said, the current code appears wrong if a CPU is in the
> list but does have X86_FEATURE_CORE_CAPABILITIES.  Are there such
> CPUs?  I think either the logic should be changed or a comment should
> be added.

Is it really wrong? Code check the CPUID & CORE_CAPABILTIES first and
believes what they say. Otherwise falls back to the x86_match_cpu()
list.

I don't believe we put a CPU on that list that currently says
it supports CORE_CAPABILITIES. That could theoretically change
with a microcode update. I doubt we'd waste microcode space to do
that, but if we did, I assume we'd include the split lock bit
in the newly present MSR. So behavior would not change.

-Tony