linux-kernel - Re: [RFC PATCH] x86: Remove X86_FEATURE_MFENCE

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Date:   Wed, 10 Jul 2019 13:33:48 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paolo Bonzini <pbonzini@...hat.com>
cc:     "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Pu Wen <puwen@...on.cn>, Borislav Petkov <bp@...en8.de>
Subject: Re: [RFC PATCH] x86: Remove X86_FEATURE_MFENCE_RDTSC

On Wed, 10 Jul 2019, Paolo Bonzini wrote:
> On 08/07/19 21:32, Lendacky, Thomas wrote:
> >> AMD and Intel both have serializing lfence (X86_FEATURE_LFENCE_RDTSC).
> >> They've both had it for a long time, and AMD has had it enabled in Linux
> >> since Spectre v1 was announced.
> >>
> >> Back then, there was a proposal to remove the serializing mfence feature
> >> bit (X86_FEATURE_MFENCE_RDTSC), since both AMD and Intel have
> >> serializing lfence.  At the time, it was (ahem) speculated that some
> >> hypervisors might not yet support its removal, so it remained for the
> >> time being.
> >>
> >> Now a year-and-a-half later, it should be safe to remove.
> >
> > I vaguely remember a concern from a migration point of view, maybe? Adding
> > Paolo to see if he has any concerns.
> 
> It would be a problem to remove the conditional "if
> (boot_cpu_has(X86_FEATURE_LFENCE_RDTSC))" from svm_get_msr_feature.  But
> removing support for X86_FEATURE_MFENCE_RDTSC essentially amounts to
> removing support for hypervisors that haven't been updated pre-Spectre.
>  That's fair enough, I think.

Yes, they have other more interesting problems :)