linux-kernel - Re: New vsyscall emulation breaks JITs

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Date:	Fri, 5 Aug 2011 17:26:18 -0400
From:	Andrew Lutomirski <luto@....edu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	lueckintel@...oo.com, kimwooyoung@...il.com
Subject: Re: New vsyscall emulation breaks JITs

On Fri, Aug 5, 2011 at 5:21 PM, Andi Kleen <andi@...stfloor.org> wrote:
> On Fri, Aug 05, 2011 at 05:00:44PM -0400, Andrew Lutomirski wrote:
>> > If anything runtime, but really for me it looks like the vsyscall
>> > changes should be only in one of those limited compability paranoia
>> > patchkits.
>>
>> Switching it in runtime would be a giant mess because user code might
>
> You can always switch at boot time.

For a boot time switch, it might be nicer to just switch between the
current int 0xcc sequence and the older

mov cx, 0x<magic>
int 0xcc
ret

sequence.

That way there's a ret in the vsyscall page but no syscall instruction.

--Andy
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