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

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Date:	Tue, 19 Jan 2010 12:10:17 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Peter Zijlstra <peterz@...radead.org>, kvm@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, avi@...hat.com,
	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 01/19/2010 09:44 AM, Gleb Natapov wrote:
> 
> Yes it can be done this way and I'll look into it once more. Using
> exception vector is more convenient for three reasons: it allows to pass
> additional data in error code, it doesn't require guest to issue EOI,
> exception can be injected when interrupts are disabled by a guest. The
> last one is not important for now since host doesn't inject notifications
> when interrupts are disabled currently. Having Intel allocate one
> exception vector for hypervisor use would be really nice though.
> 

That's probably not going to happen, for the rather obvious reason: *you
already have 224 of them*.

You seem to be thinking here that vectors 0-31 have to be exceptions and
32-255 have to be interrupts.  *There is no such distinction*; the only
thing special about 0-31 is that we (Intel) reserve the right to control
the assignments; for 32-255 the platform and OS control the assignment.

You can have the guest OS take an exception on a vector above 31 just
fine; you just need it to tell the hypervisor which vector it, the OS,
assigned for this purpose.

	-hpa
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