linux-kernel - Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re: [RFC] weird crap with vdso on uml/i386)

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Message-ID: <20110822095336.GB25949@kernel.org>
Date:	Mon, 22 Aug 2011 11:53:37 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andrew Lutomirski <luto@....edu>, Borislav Petkov <bp@...64.org>,
	user-mode-linux-devel@...ts.sourceforge.net,
	Richard Weinberger <richard@....at>,
	linux-kernel@...r.kernel.org, mingo@...hat.com,
	Al Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re:
 [RFC] weird crap with vdso on uml/i386)


* H. Peter Anvin <hpa@...or.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
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