linux-kernel - Re: [PATCH v15] x86/split_lock: Enable split lock detection by kernel

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Date:   Sat, 25 Jan 2020 13:42:32 -0800
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, 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 v15] x86/split_lock: Enable split lock detection by kernel

On Sat, Jan 25, 2020 at 09:33:12PM +0100, Borislav Petkov wrote:
> On Sat, Jan 25, 2020 at 09:12:21PM +0100, Peter Zijlstra wrote:
> > My thinking was Virt, virt likes to mess up all msr expectations.
> 
> My only worry is to have it written down why we're doing this so that it
> can be changed/removed later, when we've forgotten all about split lock.
> Because pretty often we look at a comment-less chunk of code and wonder,
> "why the hell did we add this in the first place."

Ok. I added a comment:

 * Use the "safe" versions of rdmsr/wrmsr here because although code
 * checks CPUID and MSR bits to make sure the TEST_CTRL MSR should
 * exist, there may be glitches in virtualization that leave a guest
 * with an incorrect view of real h/w capabilities.

-Tony