linux-kernel - Re: New vsyscall emulation breaks JITs

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Message-ID: <4E3C8B8A.3080007@zytor.com>
Date:	Fri, 05 Aug 2011 17:32:10 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andrew Lutomirski <luto@....edu>
CC:	Andi Kleen <andi@...stfloor.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	lueckintel@...oo.com, kimwooyoung@...il.com,
	Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: New vsyscall emulation breaks JITs

On 08/05/2011 05:20 PM, Andrew Lutomirski wrote:
> 
> I was thinking of 0x20 - 0x39.  0x40, 0x41, and 0x42 should do the
> trick.  I'll cook up a patch.
> 
> If you want to keep those vectors available for devices as well, we
> could hook do_general_protection instead, but that's a little messy.
> Are there x86 machines out there that are starved for interrupt
> vectors?
> 

Yes, but 3 aren't going to matter much.

However, on systems which have interrupt migration enabled we're not
using 0x21-0x2f for anything (because we need a single interrupt with
absolutely lowest priority).  Out of that range, there are a couple of
values which should be safe to use because they would be harmless
instructions of various forms:

	0x24	- AND AL, imm8
	0x25	- AND EAX, imm32
	0x26	- ES:
	0x2C	- SUB AL, imm8
	0x2D	- SUB EAX, imm32
	0x2E	- CS:

[Cc: Suresh who is the expert on the interrupt assignments]

	-hpa
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