linux-kernel - Re: [Xen-devel] [PATCH 10/24] xen: mask XSAVE from cpuid

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Message-ID: <49BD4CE1.6040100@zytor.com>
Date:	Sun, 15 Mar 2009 11:45:53 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Jan Beulich <jbeulich@...ell.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 10/24] xen: mask XSAVE from cpuid

Jeremy Fitzhardinge wrote:
> Jan Beulich wrote:
>> As pointed out on an earlier thread, it seems inappropriate to do probing
>> like this when there is a cpuid feature flag (osxsave) that can be
>> used to
>> determine whether XSAVE can be used. And even without that flag,
>> simply reading CR4 and checking whether osxsave is set there would
>> suffice. This is under the assumption that Xen's to-be-done
>> implementation
>> of XSAVE support would match that of FXSAVE (Xen turns its support on
>> unconditionally and for all [pv] guests).
> 
> I didn't want to make too many assumptions about how Xen's XSAVE support
> would look.  In particular, I thought it might virtualize the state of
> OSXSAVE to give the guest the honour of appearing to enable it.  A guest
> kernel may get confused if it starts with OSXSAVE set, as it may use it
> to control its own init logic.

That wouldn't be an issue if you use the *native* CPUID to look for
OSXSAVE early on, since such virtualization would only be visible though
the PV interface, right?

It seems cleaner than probing, to be sure...

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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