linux-kernel - Re: [PATCH 1/3] Introduce cpuid_on_cpu() and cpuid_eax_on

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Message-ID: <46112D40.20508@zytor.com>
Date:	Mon, 02 Apr 2007 09:20:16 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <ak@...e.de>
CC:	Alexey Dobriyan <adobriyan@...ru>, akpm@...l.org,
	linux-kernel@...r.kernel.org, devel@...nvz.org,
	cpufreq@...ts.linux.org.uk, davej@...emonkey.org.uk
Subject: Re: [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 
levels.

	-hpa
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