linux-kernel - Re: Use CPUID to communicate with the hypervisor.

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Message-ID: <48E4B5E8.60400@redhat.com>
Date:	Thu, 02 Oct 2008 14:52:08 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Zachary Amsden <zach@...are.com>
CC:	Gerd Hoffmann <kraxel@...hat.com>,
	Alok Kataria <akataria@...are.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Daniel Hecht <dhecht@...are.com>,
	"Jun.Nakajima@...el.Com" <Jun.Nakajima@...el.Com>
Subject: Re: Use CPUID to communicate with the hypervisor.

Zachary Amsden wrote:
>> Well, that should be clearly defined, that is my point.  When asking the
>> hypervisor for the tsc instead of running a calibration loop, then we
>> have a small bit of paravirtualization:  The guest is aware that it runs
>> on a hypervisor and just asks it directly.  So while we are at it we can
>> also define a way to communicate tsc freq changes between host and
>> guest, so the cost of trap'n'emulate tsc reads can be avoided.  Or we
>> define "tsc is constant" and leave it to the hypervisor to make sure it
>>     
>
> For our purposes, we define TSC is constant.
>   

I believe VMware doesn't actually change cpu frequency dynamically.  But 
what about hypervisors that do?  and what about large machines, which do 
not actually have a constant tsc?

You are defining something as constant which in fact is not constant.

-- 
error compiling committee.c: too many arguments to function

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