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

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Date:	Tue, 18 Jul 2006 04:33:52 -0700
From:	Zachary Amsden <zach@...are.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Chris Wright <chrisw@...s-sol.org>, 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>,
	Ian Pratt <ian.pratt@...source.com>,
	Christian Limpach <Christian.Limpach@...cam.ac.uk>
Subject: Re: [RFC PATCH 18/33] Subarch support for CPUID instruction

Arjan van de Ven wrote:
> 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.
>   

You really need a CPUID hook.  The instruction is non-virtualizable, and 
anything claiming to be a hypervisor really has to support masking and 
flattening the cpuid namespace for the instruction itself.  It is used 
in assembler code and very early in boot.  The alternative is injecting 
a bunch of Xen-specific code to filter feature bits into the i386 layer, 
which is both bad for Linux and bad for Xen - and was quite ugly in the 
last set of Xen patches.

Zach
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