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: <20110822144051.GD2946@aftab>
Date:	Mon, 22 Aug 2011 16:40:51 +0200
From:	Borislav Petkov <bp@...64.org>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Borislav Petkov <bp@...64.org>,
	"user-mode-linux-devel@...ts.sourceforge.net" 
	<user-mode-linux-devel@...ts.sourceforge.net>,
	Richard Weinberger <richard@....at>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <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)

On Mon, Aug 22, 2011 at 09:34:27AM -0400, Andrew Lutomirski wrote:
> On Mon, Aug 22, 2011 at 5:53 AM, Ingo Molnar <mingo@...nel.org> wrote:
> >
> > * 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.
> 
> From memory, SYSCALL in 64-bit mode on Sandy Bridge is much faster
> than int 0xcc, which is presumably about the same speed as int 0x80.
> That's because SYSCALL is blazingly fast (<30 ns for a whole system
> call) and int is slower.

Just to make sure I'm grokking this correctly - we want to use int $0x80
only for the SYSCALL variant in __kernel_vsyscall, right? Not for all
32-bit syscalls on a 64-bit kernel.

Thanks.

-- 
Regards/Gruss,
Boris.

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