linux-kernel - Re: [PATCH 02/11] Add "handle page fault" PV helper.

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Message-ID: <4AEFB823.4040607@redhat.com>
Date:	Tue, 03 Nov 2009 06:57:07 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Rik van Riel <riel@...hat.com>
CC:	Ingo Molnar <mingo@...e.hu>, Gleb Natapov <gleb@...hat.com>,
	kvm@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 02/11] Add "handle page fault" PV helper.

On 11/03/2009 01:35 AM, Rik van Riel wrote:
>> We can't add an exception vector since all the existing ones are either
>> taken or reserved.
>
>
> I believe some are reserved for operating system use.

Table 6-1 says:

   9 |  | Coprocessor Segment Overrun (reserved)  |  Fault |  No  | 
Floating-point instruction.2
   15 |  — |  (Intel reserved. Do not use.) |   | No |
   20-31 |  — | Intel reserved. Do not use.  |
   32-255 |  —  | User Defined (Non-reserved) Interrupts |  Interrupt  
|   | External interrupt or INT n instruction.

So we can only use 32-255, but these are not fault-like exceptions that 
can be delivered with interrupts disabled.

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

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