linux-kernel - Re: [kvm-devel] [PATCH] Refactor hypercall infrastructure

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Date:	Sat, 15 Sep 2007 11:08:44 +0300
From:	Avi Kivity <avi@...ranet.com>
To:	Zachary Amsden <zach@...are.com>
CC:	Anthony Liguori <anthony@...emonkey.ws>,
	kvm-devel@...ts.sourceforge.net,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [kvm-devel] [PATCH] Refactor hypercall infrastructure

Zachary Amsden wrote:
> On Fri, 2007-09-14 at 16:44 -0500, Anthony Liguori wrote:
>
>   
>> So then each module creates a hypercall page using this magic MSR and 
>> the hypervisor has to keep track of it so that it can appropriately 
>> change the page on migration.  The page can only contain a single 
>> instruction or else it cannot be easily changed (or you have to be able 
>> to prevent the guest from being migrated while in the hypercall page).
>>
>> We're really talking about identical models.  Instead of an MSR, the #GP 
>> is what tells the hypervisor to update the instruction.  The nice thing 
>> about this is that you don't have to keep track of all the current 
>> hypercall page locations in the hypervisor.
>>     
>
> I agree, multiple hypercall pages is insane.  I was thinking more of a
> single hypercall page, fixed in place by the hypervisor, not the kernel.
>
> Then each module can read an MSR saying what VA the hypercall page is
> at, and the hypervisor can simply flip one page to switch architectures.
>   

VA as in "Virtual Address"?  the ppc people don't have
hypervisor-visible virtual addresses, and the hypervisor (on x86) can't
safely select a virtual address, and ...

That means you need a physical address, so you need a central
initialization routine, and drivers for unmodified OSes can no longer be
self contained.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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