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

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Message-Id: <200906100655.29833.lkml@morethan.org>
Date:	Wed, 10 Jun 2009 06:55:27 -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:
> 
> * Michael S. Zick <lkml@...ethan.org> wrote:
> 
> > 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.
> 
> these files are accessed via read(). The -EINVAL is the syscall 
> return value. The value (if any) goes into the buffer that is being 
> read into. So there's no way to 'match a specific bit pattern' - 
> it's two separate spaces.
> 

Right.  Prior post was just a bad case of nerves.  ;)

Mike
> 	Ingo
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