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

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Date:   Wed, 10 Jul 2019 08:20:40 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Thomas Gleixner <tglx@...utronix.de>, Pu Wen <puwen@...on.cn>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [RFC PATCH] x86: Remove X86_FEATURE_MFENCE_RDTSC

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.

Paolo