Re: [PATCH v6 1/4] x86: Clean up extable entry format (and free up a bit)

[Tony Luck <tony.luck@gmail.com>]

> ----- begin comment -----
>
> The offset to the fixup is signed, and we're trying to use the high
> bits for a different purpose.  In C, we could just do:
>
> u32 class_and_offset = ((target - here) & 0x3fffffff) | class;
>
> Then, to decode it, we'd mask off the class and sign-extend to recover
> the offset.
>
> In asm, we can't do that, because this all gets laundered through the
> linker, and there's no relocation type that supports this chicanery.
> Instead we cheat a bit.  We first add a large number to the offset
> (0x20000000).  The result is still nominally signed, but now it's
> always positive, and the two high bits are always clear.  We can then
> set high bits by ordinary addition or subtraction instead of using
> bitwise operations.  As far as the linker is concerned, all we're
> doing is adding a large constant to the difference between here (".")
> and the target, and that's a valid relocation type.
>
> In the C code, we just mask off the class bits and subtract 0x20000000
> to get the offset.
>
> ----- end comment -----

But presumably those constants get folded together, so the linker
is dealing with only one offset.  It doesn't (I assume) know that our
source code added 0x20000000 and then added/subtracted some
more.

It looks like we could just use:
class0: +0x40000000
class1: +0x80000000 (or subtract ... whatever doesn't make the linker cranky)
class2: -0x40000000
class3: don't add/subtract anything

ex_class() stays the same (just looks at bit31/bit30)
ex_fixup_addr() has to use ex_class() to decide what to add/subtract
(if anything).

Would that work?  Would it be more or less confusing?

-Tony

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