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: <CAObL_7F6D8nQovf+aJGiEOF1mBpBU8RwtjNA-5myxvEBRRHgsg@mail.gmail.com>
Date:	Mon, 22 Aug 2011 09:34:27 -0400
From:	Andrew Lutomirski <luto@....edu>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, 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)

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.

--Andy

>
> 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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