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: <4E53FC6E.1030807@zytor.com>
Date:	Tue, 23 Aug 2011 12:15:58 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...64.org>
CC:	Andrew Lutomirski <luto@....edu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Ingo Molnar <mingo@...nel.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>
Subject: Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re:
 [RFC] weird crap with vdso on uml/i386)

On 08/23/2011 09:22 AM, Borislav Petkov wrote:
> On Tue, Aug 23, 2011 at 12:11:43PM -0400, Andrew Lutomirski wrote:
>> In any case, this seems insanely overcomplicated.  I'd be less afraid
>> of something like my approach (which, I think, makes all of the
>> SYSCALL weirdness pretty much transparent to ptrace users) or of just
>> removing SYSCALL entirely from 32-bit code.
> 
> I don't think that removing SYSCALL from 32-bit code just so that UML
> trapped syscalls work is something we'd like since SYSCALL is much
> cheaper than INT $0x80:
> 
> "As a result, SYSCALL and SYSRET can take fewer than one-fourth the
> number of internal clock cycles to complete than the legacy CALL and RET
> instructions."
> 
> http://support.amd.com/us/Processor_TechDocs/24593.pdf, p. 152.
> 
> I know, it is 32-bit syscall on 64-bit kernel which should be pretty
> rare but still...
> 

Right, but if you had said the difference had disappeared on current AMD
silicon it would be much less of an issue.  That's why I wanted to find
that bit out from you.
	
	-hpa
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