linux-kernel - Re: [tip:x86/pti] x86/cpu/AMD: Use LFENCE_RDTSC instead of MFENCE

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Date:   Mon, 8 Jan 2018 11:08:55 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     bp@...en8.de, dwmw@...zon.co.uk, gregkh@...ux-foundation.org,
        thomas.lendacky@....com, pjt@...gle.com, mingo@...nel.org,
        linux-kernel@...r.kernel.org, hpa@...or.com,
        tim.c.chen@...ux.intel.com, torvalds@...ux-foundation.org,
        peterz@...radead.org, dave.hansen@...el.com
cc:     linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/pti] x86/cpu/AMD: Use LFENCE_RDTSC instead of
 MFENCE_RDTSC

On Sat, 6 Jan 2018, tip-bot for Tom Lendacky wrote:

> Commit-ID:  0bf17c102177d5da9363bf8b1e4704b9996d5079
> Gitweb:     https://git.kernel.org/tip/0bf17c102177d5da9363bf8b1e4704b9996d5079
> Author:     Tom Lendacky <thomas.lendacky@....com>
> AuthorDate: Fri, 5 Jan 2018 10:07:56 -0600
> Committer:  Thomas Gleixner <tglx@...utronix.de>
> CommitDate: Sat, 6 Jan 2018 21:57:40 +0100
> 
> x86/cpu/AMD: Use LFENCE_RDTSC instead of MFENCE_RDTSC
> 
> With LFENCE now a serializing instruction, set the LFENCE_RDTSC
> feature since the LFENCE instruction has less overhead than the
> MFENCE instruction.

Second thoughts on that. As pointed out by someone in one of the insane
long threads:

What happens if the kernel runs as a guest and

  - the hypervisor did not set the LFENCE to serializing on the host

  - the hypervisor does not allow writing MSR_AMD64_DE_CFG

That would bring the guest into a pretty bad state or am I missing
something essential here?

I'm dropping these patches until this question is answered.

Thanks,

	tglx