linux-kernel - Re: [PATCH v3 04/12] Add "handle page fault" PV helper.

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Date:	Thu, 21 Jan 2010 07:47:22 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Gleb Natapov <gleb@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>, kvm@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, mingo@...e.hu,
	tglx@...utronix.de, riel@...hat.com, cl@...ux-foundation.org
Subject: Re: [PATCH v3 04/12] Add "handle page fault" PV helper.

On 01/21/2010 01:02 AM, Avi Kivity wrote:
>>
>> You can also just emulate the state transition -- since you know
>> you're dealing with a flat protected-mode or long-mode OS (and just
>> make that a condition of enabling the feature) you don't have to deal
>> with all the strange combinations of directions that an unrestricted
>> x86 event can take.  Since it's an exception, it is unconditional.
> 
> Do you mean create the stack frame manually?  I'd really like to avoid
> that for many reasons, one of which is performance (need to do all the
> virt-to-phys walks manually), the other is that we're certain to end up
> with something horribly underspecified.  I'd really like to keep as
> close as possible to the hardware.  For the alternative approach, see Xen.
> 

I obviously didn't mean to do something which didn't look like a
hardware-delivered exception.  That by itself provides a tight spec.
The performance issue is real, of course.

Obviously, the design of VT-x was before my time at Intel, so I'm not
familiar with why the tradeoffs that were done they way they were.

	-hpa

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

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