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

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Date:   Mon, 8 Jan 2018 12:10:33 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
cc:     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,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/pti] x86/cpu/AMD: Use LFENCE_RDTSC instead of
 MFENCE_RDTSC

On Mon, 8 Jan 2018, Andrew Cooper wrote:

> On 08/01/18 10:08, Thomas Gleixner wrote:
> > 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?
> 
> What I did in Xen was to attempt to set it, then read it back and see. 
> If LFENCE still isn't serialising, using repoline is the only available
> mitigation.
> 
> My understanding from the folk at AMD is that retpoline is safe to use,
> but has higher overhead than the LFENCE approach.

That still does not help vs. rdtsc_ordered() and LFENCE_RDTSC ...

Thanks,

	tglx