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From: Andrew Lutomirski <luto@mit.edu>
Date: Tue, 31 May 2011 09:06:45 -0400
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Subject: Re: [PATCH 0/5] x86-64: Remove syscall instructions at fixed addresses
To: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
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On Tue, May 31, 2011 at 8:54 AM, Ingo Molnar <mingo@elte.hu> wrote:
>
> * Andrew Lutomirski <luto@mit.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
>