linux-kernel - Re: build fix for x86

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Message-ID: <46A0F4AC.50809@zytor.com>
Date:	Fri, 20 Jul 2007 10:45:16 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andreas Schwab <schwab@...e.de>
CC:	"Luck, Tony" <tony.luck@...el.com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Andi Kleen <ak@...ell.com>,
	Arthur Jones <arthur.jones@...gic.com>,
	Vasily Tarasov <vtaras@...nvz.org>,
	linux-kernel@...r.kernel.org, Jan Kara <jack@....cz>,
	linux-arch@...r.kernel.org
Subject: Re: build fix for x86_64...

Andreas Schwab wrote:
> "H. Peter Anvin" <hpa@...or.com> writes:
> 
>> No, that would be bad.  If compat_u64 is used to carry 32-bit ABIs
>> forward into 64-bit space without needing compatibility hacks, then this
>> would actually introduce ABI incompatibilities depending on CONFIG_COMPAT!
> 
> But without CONFIG_COMPAT there is no 32-bit ABI, thus no need for
> compat_u64 in the first place.
> 

You're missing the point.

Someone introduces an interface, which uses a structure:

struct foo {
	u32 bar;
	u64 baz;
	u32 quux;
};

Now, we want to port that to 64 bits.  We can either introduce a
thunking function to mangle the argument, or we can redefine the structure:

struct foo {
	u32 bar;
	compat_u64 baz;
	u32 quux;
};

... which is still ABI compatible on 32 bits, but doesn't require thunking.

Obviously, this is not a panacea; if the original "struct foo" has also
been introduced on 64 bits before the bug is caught, then you're screwed.

	-hpa

	
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