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    SubjectRe: [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@amd.com>
    > AuthorDate: Fri, 5 Jan 2018 10:07:56 -0600
    > Committer: Thomas Gleixner <tglx@linutronix.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

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