linux-kernel - Re: [RFC] CPUID usage for interaction between Hypervisors and Linux.

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Message-ID: <48E3ECD1.30809@codemonkey.ws>
Date:	Wed, 01 Oct 2008 16:34:09 -0500
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	akataria@...are.com, "avi@...hat.com" <avi@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Gerd Hoffmann <kraxel@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Nakajima, Jun" <jun.nakajima@...el.com>,
	Daniel Hecht <dhecht@...are.com>,
	Zach Amsden <zach@...are.com>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [RFC] CPUID usage for interaction between Hypervisors and	Linux.

Jeremy Fitzhardinge wrote:
> Alok Kataria wrote:
>
> I guess, but the bulk of the uses of this stuff are going to be 
> hypervisor-specific.  You're hard-pressed to come up with any other 
> generic uses beyond tsc.

And arguably, storing TSC frequency in CPUID is a terrible interface 
because the TSC frequency can change any time a guest is entered.  It 
really should be a shared memory area so that a guest doesn't have to 
vmexit to read it (like it is with the Xen/KVM paravirt clock).

Regards,

Anthony Liguori

>   In general, if a hypervisor is going to put something in a special 
> cpuid leaf, its because there's no other good way to represent it.  
> Generic things are generally going to appear as an emulated piece of 
> the virtualized platform, in ACPI, DMI, a hardware-defined cpuid leaf, 
> etc...

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