Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 13 Nov 2017 10:06:47 -0800 | Subject | Re: Can we break RDPID/RDTSCP ABI ASAP and see if it's okay? |
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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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