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    SubjectRe: [PATCH 1/3] Introduce cpuid_on_cpu() and cpuid_eax_on_cpu()
    Andi Kleen wrote:
    > On Monday 02 April 2007 13:38, Alexey Dobriyan wrote:
    >> They will be used by cpuid driver and powernow-k8 cpufreq driver.
    >> With these changes powernow-k8 driver could run correctly on OpenVZ kernels
    >> with virtual cpus enabled (SCHED_VCPU).
    > This means openvz has multiple virtual CPU levels? One for cpuid/rdmsr and one
    > for the rest of the kernel? Both powernow-k8 and cpuid attempt to schedule
    > to the target CPU so they should already run there. But it is some other CPU,
    > but when they ask your _on_cpu() functions they suddenly get a "real" CPU?
    > Where is the difference between these levels of virtualness?

    The CPUID and MSR drivers do not schedule to the target CPU; instead, on
    hardware, they rely on IPI'ing the target processor if it is not the one
    that's currently running.

    There were a lot of discussion back when about which was the better
    solution. Alan Cox, in particular, really preferred the interrupt
    solution as being less likely to cause implicit deadlock.

    I do want to add that it's been on my list for some time -- in fact, I
    keep implementing it half-way and then having other things to do -- to
    add MSR and CPUID ioctls() that allow the full register file to be set
    and read back, in order to support architecturally broken MSR and CPUID

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