linux-kernel - Re: [PATCH 0/5] x86-64: Remove syscall instructions at fixed addresses

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Message-ID: <BANLkTinqGUoFTp3YcuUTAPUOUwBCQaw07g@mail.gmail.com>
Date:	Tue, 31 May 2011 09:06:45 -0400
From:	Andrew Lutomirski <luto@....edu>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Jan Beulich <JBeulich@...ell.com>
Subject: Re: [PATCH 0/5] x86-64: Remove syscall instructions at fixed addresses

On Tue, May 31, 2011 at 8:54 AM, Ingo Molnar <mingo@...e.hu> wrote:
>
> * Andrew Lutomirski <luto@....edu> wrote:
>
>> [...] More importantly, vread_tsc contains an alternative and the
>> vDSO can't currently contain alternative instructions.  That can
>> probably be fixed, but it'll take a bit of work.
>
> You could start with picking the more compatible alternative
> instruction initially. I don't at all mind losing half a cycle of
> performance in that case ... this code should be secure first.

The more compatible one is mfence, which in some cases could (I think)
be a lot more than half a cycle.

A better option might be rdtscp, which is actually documented to work,
but I'm not sure it's available on all supported CPUs.

I'm content to wait a bit on this one.  I say let's get the rest done
first and tackle the last little hard part at the end.

--Andy

>
> Thanks,
>
>        Ingo
>
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