linux-kernel - Re: [perfmon2] I.1 - System calls

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Date:	Mon, 13 Jul 2009 19:53:45 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	perfmon2-devel@...ts.sourceforge.net,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>, eranian@...il.com,
	Philip Mucci <mucci@...s.utk.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Paul Mackerras <paulus@...ba.org>,
	Maynard Johnson <mpjohn@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [perfmon2] I.1 - System calls - ioctl

On Monday 13 July 2009, Peter Zijlstra wrote:
> On Mon, 2009-07-13 at 19:30 +0200, Arnd Bergmann wrote:
> > > +struct perf_counter_target {
> > > +	__u32			id;
> > > +	__u64			val;
> > > +};
> > 
> > This structure is not compatible between 32 and 64 bit user space on x86,
> > because everything except i386 adds implicit padding between id and val.
> 
> Humm, __u64 doesn't have natural alignment? That would break more than
> just this I think -- it sure surprises me.

Yes, nobody expects this, so it is a frequent source of bugs in the ABI.
Look for compat_u64 and __packed in the definition of compat ioctl and
syscall interfaces for how we had to work around this elsewhere.

> > Other than that, making it extensible sounds reasonable. How about just
> > using a '__u64 *target' and a bit in the 'flags' argument?
> 
> Would there still be a point in having it a pointer in that case?, but
> yeah, that might work too?

passing u64 bit arguments directly to system calls is a bit complicated,
because some 32 bit architectures can only pass them in certain
register pairs, see the trouble we go through for llseek, sync_file_range
or preadv.

If you can directly pass an 'unsigned long' instead, that would work fine
though.

	Arnd <><
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