linux-kernel - Re: [PATCH] X86: cpu_debug support for VIA / Centaur CPU's

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Message-Id: <200906100611.14941.lkml@morethan.org>
Date:	Wed, 10 Jun 2009 06:11:10 -0500
From:	"Michael S. Zick" <lkml@...ethan.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"H. Peter Anvin" <hpa@...nel.org>,
	Harald Welte <HaraldWelte@...tech.com>,
	Jaswinder Singh Rajput <jaswinder@...nel.org>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de
Subject: Re: [PATCH] X86: cpu_debug support for VIA / Centaur CPU's

On Wed June 10 2009, Ingo Molnar wrote:
> 
> * H. Peter Anvin <hpa@...nel.org> wrote:
> 
> > Ingo Molnar wrote:
> > > 
> > > MSRs should really be enumerated along CPU features. They will be 
> > > accessed if a CPU offers that CPU feature.
> > > 
> > 
> > Nice in theory, but so many MSRs have to be enumerated with obscure test
> > combinations, that it really isn't practical in the general case.  That
> > is why we have the safe MSR variants.
> > 
> > > 
> > > Yeah, the safe read should never fault - there should be all 
> > > zeroes or an error return.
> > > 
> > 
> > Error return, MSRs #GP if not present.  All zero means a present 
> > MSR (which is zero.)
> 
> yes, of course - i meant the /debug/x86/cpu/* behavior: it should 
> either result zeroes, or should return -EINVAL. (probably the 
> latter)
> 

Return zeroes - same as hardware case for bits which can't be set.
Returning -EINVAL might match a specific bit pattern caller is looking for.

Mike
> 	Ingo
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