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Subject: Re: [RFC PATCH] x86: Remove X86_FEATURE_MFENCE_RDTSC
To:     "Lendacky, Thomas" <Thomas.Lendacky@amd.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        "x86@kernel.org" <x86@kernel.org>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Thomas Gleixner <tglx@linutronix.de>, Pu Wen <puwen@hygon.cn>,
        Borislav Petkov <bp@alien8.de>
References: <d990aa51e40063acb9888e8c1b688e41355a9588.1562255067.git.jpoimboe@redhat.com>
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From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <4a13c6a3-a13e-d3e5-0008-41a6d47a6eff@redhat.com>
Date:   Wed, 10 Jul 2019 08:20:40 +0200
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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