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Date:   Sat, 25 Jan 2020 13:42:32 -0800
From:   "Luck, Tony" <tony.luck@intel.com>
To:     Borislav Petkov <bp@alien8.de>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        "Christopherson, Sean J" <sean.j.christopherson@intel.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Yu, Fenghua" <fenghua.yu@intel.com>,
        Ingo Molnar <mingo@redhat.com>, H Peter Anvin <hpa@zytor.com>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
Subject: Re: [PATCH v15] x86/split_lock: Enable split lock detection by kernel
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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