Messages in this thread | | | Date | Mon, 02 Apr 2007 09:20:16 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH 1/3] Introduce cpuid_on_cpu() and cpuid_eax_on_cpu() |
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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 levels.
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