linux-kernel - Re: [PATCH] x86, msr: add support for non-contiguous cpumasks

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Date:	Fri, 11 Dec 2009 08:30:39 +0100
From:	Borislav Petkov <borislav.petkov@....com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Mauro Carvalho Chehab <mchehab@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Aristeu Rozanski <aris@...hat.com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Doug Thompson <norsk5@...oo.com>, linux-kernel@...r.kernel.org,
	x86 <x86@...nel.org>
Subject: Re: [PATCH] x86, msr: add support for non-contiguous cpumasks

On Thu, Dec 10, 2009 at 05:49:02PM -0800, H. Peter Anvin wrote:
> On 12/07/2009 04:21 AM, Borislav Petkov wrote:
> >  
> >  struct msr {
> > +	int cpu;
> >  	union {
> >  		struct {
> >  			u32 l;
> 
> I really don't like this patch, for multiple reasons.  One of them is
> the above: this has no business being part of struct msr, which reflects
> an MSR value (and ideally should replace at least the use of two
> pointers in other places over time).  Having a CPU field and not an MSR
> number field particular doesn't make any sense.

Why, MSRs are per-CPU. My reasoning here is to reflect the value of an
MSR on a particular CPU...

[..]

> The ideal probably would be to use a percpu variable.  Now, this would
> either have to be a dynamic percpu allocation (which would have to be
> the responsibility of the caller, and reused, lest this would be a
> *very* expensive proposition), or we would have to make these functions
> run under a mutex.  However, as long as the expected callers of this are
> things that get set up once and then pretty much stick around, a percpu
> variable might just work.

I think this would be the cleanest way. Also, best it'll be to allocate
those dynamically only when they're really needed (e.g. on driver
loading) and later reuse them.

[..]

> The third option would be to at least require that the struct msr
> contents are at least serial in the order of the bitmask, not by adding
> another field.

I had that version already done but it seemed half-baked and clumsy for
the MSRs array to traverse. I'll give the percpu variables a shot and
get back to you when I have something ready.

Thanks for reviewing.

-- 
Regards/Gruss,
Boris.

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