Re: build fix for x86_64...

[H. Peter Anvin]

Andreas Schwab wrote:
> "H. Peter Anvin" <hpa@xxxxxxxxx> 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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