Re: [Xen-devel] [PATCH 5/7] x86/alt: Support for automatic padding calculations

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Feb 12, 2018 at 11:23:05AM +0000, Andrew Cooper wrote:
>  .macro ALTERNATIVE oldinstr, newinstr, feature
>  .L\@_orig_s:
>      \oldinstr
>  .L\@_orig_e:
> +     .skip (-((repl_len(1) - orig_len) > 0) * (repl_len(1) - orig_len)), 0x90

clang chokes on this expression, because of the negation at the
beginning and I'm also failing to see why are you adding such
negation. AFAICT using:

.skip (((repl_len(1) - orig_len) > 0) * (repl_len(1) - orig_len)), 0x90

Is correct: it adds the right padding if the alternative code is
bigger than the original one, while not adding anything is the
original code is greater than the alternative one.

The negation just turns the 1 to -1, thus converting the result of the
whole expression into a negative value.

Roger.

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