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Subject: RE: Intel P6 vs P7 system call performance
Date: Sun, 22 Dec 2002 07:45:18 -0800
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From: "Nakajima, Jun" <jun.nakajima@intel.com>
To: "Mikael Pettersson" <mikpe@csd.uu.se>, <mingo@elte.hu>,
       <torvalds@transmeta.com>
Cc: <drepper@redhat.com>, <linux-kernel@vger.kernel.org>
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Correct. Please look at Table B-1. Most of MSRs are shared, but some MSRs are unique in each logical processor, to provide the x86 architectural state. Those SYSENTER MSRs, and Machine Check register save state (IA32_MCG_XXX), for example, are unique.

Jun

> -----Original Message-----
> From: Mikael Pettersson [mailto:mikpe@csd.uu.se]
> Sent: Sunday, December 22, 2002 4:34 AM
> To: mingo@elte.hu; torvalds@transmeta.com
> Cc: drepper@redhat.com; Nakajima, Jun; linux-kernel@vger.kernel.org
> Subject: Re: Intel P6 vs P7 system call performance
> 
> On Sun, 22 Dec 2002 11:23:08 +0100 (CET), Ingo Molnar wrote:
> >while reviewing the sysenter trampoline code i started wondering about
> the
> >HT case. Dont HT boxes share the MSRs between logical CPUs? This pretty
> >much breaks the concept of per-logical-CPU sysenter trampolines. It also
> >makes context-switch time sysenter MSR writing impossible, so i really
> >hope this is not the case.
> 
> Some MSRs are shared, some aren't. One must always check this in
> the IA32 Volume 3 manual. The three SYSENTER MSRs are not shared.
> 
> However, no-one has yet proven that writing to these in the context
> switch path has acceptable performance -- remember, there is _no_
> a priori reason to assume _anything_ about performance on P4s,
> you really do need to measure things before taking design decisions.
> 
> Manfred had a version with fixed MSR values and the varying data
> in memory. Maybe that's actually faster.
-
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