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    SubjectRe: Can we break RDPID/RDTSCP ABI ASAP and see if it's okay?
    On Mon, Nov 13, 2017 at 5:54 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
    > On 28/10/2017 21:38, Andy Lutomirski wrote:
    >> We currently do this on boot:
    >>
    >> write_rdtscp_aux((node << 12) | cpu);
    >>
    >> This *sucks*. It means that, to very quickly obtain the CPU number
    >> using RDPID, an ALU op is needed. It also doesn't bloody work on
    >> systems with more than 4096 CPUs.
    >>
    >> IMO it should be ((u64)node << 32) | cpu.
    >
    > MSR_TSC_AUX is still documented as reserving bits 32-63 in the October
    > 2017 SDM, and indeed on a Haswell you get a #GP if you write a nonzero
    > value to it.
    >
    > Has Intel quietly "unreserved" them on machines that have RDPID?
    >

    Dunno. I don't have such a machine.

    But this is a genuine problem on machines with >4096 CPUs.

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