linux-kernel - Re: [RFC PATCH 18/33] Subarch support for CPUID instruction

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Date:	Tue, 18 Jul 2006 12:14:46 +0200
From:	Arjan van de Ven <arjan@...radead.org>
To:	Chris Wright <chrisw@...s-sol.org>
Cc:	linux-kernel@...r.kernel.org, virtualization@...ts.osdl.org,
	xen-devel@...ts.xensource.com,
	Jeremy Fitzhardinge <jeremy@...p.org>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...l.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Zachary Amsden <zach@...are.com>,
	Ian Pratt <ian.pratt@...source.com>,
	Christian Limpach <Christian.Limpach@...cam.ac.uk>
Subject: Re: [RFC PATCH 18/33] Subarch support for CPUID instruction

On Tue, 2006-07-18 at 00:00 -0700, Chris Wright wrote:
> plain text document attachment (i386-cpuid)
> Allow subarchitectures to modify the CPUID instruction.  This allows
> the subarch to provide a limited set of CPUID feature flags during CPU
> identification.  Add a subarch implementation for Xen that traps to the
> hypervisor where unsupported feature flags can be hidden from guests.

Hi,

I'm wondering if this is entirely the wrong level of abstraction; to me
it feels the subarch shouldn't override the actual cpuid, but the cpu
feature flags that linux uses. That's a lot less messy: cpuid has many
many pieces of information which are near impossible to filter in
practice, however filtering the USAGE of it is trivial; linux basically
flattens the cpuid namespace into a simple bitmap of "what the kernel
can use". That is really what the subarch should filter/fixup, just like
we do for cpu quirks etc etc.

Greetings,
   Arjan van de Ven

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