linux-kernel - Re: Various x86 syscall mechanisms

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Message-ID: <87vdzup31p.fsf@basil.nowhere.org>
Date:	Sat, 28 Jun 2008 07:00:02 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Roland McGrath <roland@...hat.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Various x86 syscall mechanisms

Roland McGrath <roland@...hat.com> writes:
>
> I think it is clearest to talk separately about the "intended ABI", the
> "what actually works today", and the "why".  (Also note I was not the
> decision-maker in this, just picking up what I can see.)

You are correct.

> For the 32-bit ABI, what I believe was always the intent for what could be
> considered the proper ABI is "int 0x80" or "use the vDSO entry point".  If
> someone asked me what you could ever have expected to rely on for the
> future, I would say exactly that.  The use of the vDSO is explicitly
> intended to take the details of sysenter/syscall or other such new
> instructions out of the 32-bit ABI picture for what any proper application
> will expect from the kernel.

For SYSENTER the vDSO is even needed because it relies on a hardcoded 
return address.

> AMD's were the first x86_64 CPUs, and those always supported "syscall"
> from 32-bit tasks to 64-bit kernels.  (I don't know whether AMD CPUs now
> support "sysenter" from 32-bit tasks to 64-bit kernels, and if so which
> past AMD64 CPUs may not have supported that.  On today's kernel you could

K8 at least.

> It was long on my back-burner list to toss in the "syscall" version of the
> 32-bit vDSO for 32-bit kernels on hardware that supports "syscall".  But,

That would only make a difference on K6 (K7 supports SYSENTER), and also
K6/K7 SYSCALL was slightly different from the K8 version.

-Andi
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