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

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Message-Id: <214b20545df13e36c91fd1369d57b18a@cl.cam.ac.uk>
Date:	Tue, 18 Jul 2006 11:26:19 +0100
From:	Keir Fraser <Keir.Fraser@...cam.ac.uk>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	virtualization@...ts.osdl.org,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	xen-devel@...ts.xensource.com, Ian Pratt <ian.pratt@...source.com>,
	linux-kernel@...r.kernel.org, Chris Wright <chrisw@...s-sol.org>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [RFC PATCH 18/33] Subarch support for CPUID instruction


On 18 Jul 2006, at 11:14, Arjan van de Ven wrote:

>> 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.
>
> 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.

Maybe we should have that *as well*, but it makes sense to allow the 
hypervisor to apply a filter too. For example, whether it supports PSE, 
FXSAVE/FXRSTOR, etc. These are things the 'platform' is telling the OS 
-- not something the OS can filter for itself. To trap on CPUID 
invocations requires the guest to use a special code sequence for 
CPUID, since the instruction will never normally fault. Hence moving to 
mach-* to hide this detail.

  -- Keir

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