linux-kernel - Re: New vsyscall emulation breaks JITs

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Date:	Tue, 09 Aug 2011 15:27:24 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andrew Lutomirski <luto@....edu>, Andi Kleen <andi@...stfloor.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"lueckintel@...oo.com" <lueckintel@...oo.com>,
	"kimwooyoung@...il.com" <kimwooyoung@...il.com>
Subject: Re: New vsyscall emulation breaks JITs

On Fri, 2011-08-05 at 17:32 -0700, H. Peter Anvin wrote:
> 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).

Double checked to make sure and we actually allow 0x21-0x2f to be used
for device interrupts (commit 6579b474572fd54c583ac074e8e7aaae926c62ef).
So reserving the vectors in this range should be same as reserving in
any other range available for use.

Thanks.

>   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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