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Date: Mon, 22 Aug 2011 11:53:37 +0200
From: Ingo Molnar <mingo@kernel.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andrew Lutomirski <luto@mit.edu>, Borislav Petkov <bp@amd64.org>,
        user-mode-linux-devel@lists.sourceforge.net,
        Richard Weinberger <richard@nod.at>, linux-kernel@vger.kernel.org,
        mingo@redhat.com, Al Viro <viro@zeniv.linux.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re:
 [RFC] weird crap with vdso on uml/i386)
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* H. Peter Anvin <hpa@zytor.com> wrote:

> Borislav,
> 
> We're tracking down an issue with the way system call arguments are 
> handled on 32 bits.  We have a solution for SYSENTER but not 
> SYSCALL; fixing SYSCALL "properly" appears to be very difficult at 
> best.
> 
> So the question is: how much overhead would it be to simply fall 
> back to int $0x80 or some other legacy-style domain crossing 
> instruction for 32-bit system calls on AMD64 processors?  We don't 
> ever use SYSCALL in legacy mode, so native i386 kernels are 
> unaffected.

Last i measured INT80 and SYSCALL costs they were pretty close to 
each other on AMD CPUs - closer than on Intel.

Also, most installations are either pure 32-bit or dominantly 64-bit, 
the significantly mixed-mode case is dwindling.

Unifying some more in this area would definitely simplify things ...

Thanks,

	Ingo