Re: [PATCH v2 10/11] x86/alternatives: Simplify ALTERNATIVE_n()

[Nikolay Borisov]

On 14.08.23 г. 14:44 ч., Peter Zijlstra wrote:
> Instead of making increasingly complicated ALTERNATIVE_n()
> implementations, use a nested alternative expression.
>
> The only difference between:
>
>    ALTERNATIVE_2(oldinst, newinst1, flag1, newinst2, flag2)
>
> and
>
>    ALTERNATIVE(ALTERNATIVE(oldinst, newinst1, flag1),
>                newinst2, flag2)
>
> is that the outer alternative can add additional padding when the
> inner alternative is the shorter one, which then results in
> alt_instr::instrlen being inconsistent.
>
> However, this is easily remedied since the alt_instr entries will be
> consecutive and it is trivial to compute the max(alt_instr::instrlen)
> at runtime while patching.
>
> Specifically, after this patch the ALTERNATIVE_2 macro, after CPP
> expansion (and manual layout), looks like this:
>
>    .macro ALTERNATIVE_2 oldinstr, newinstr1, ft_flags1, newinstr2, ft_flags2
>     140:
>
>       140: \oldinstr ;
>       141: .skip -(((144f-143f)-(141b-140b)) > 0) * ((144f-143f)-(141b-140b)),0x90 ;
>       142: .pushsection .altinstructions,"a" ;
> 	  altinstr_entry 140b,143f,\ft_flags1,142b-140b,144f-143f ;
> 	  .popsection ; .pushsection .altinstr_replacement,"ax" ;
>       143: \newinstr1 ;
>       144: .popsection ; ;
>
>     141: .skip -(((144f-143f)-(141b-140b)) > 0) * ((144f-143f)-(141b-140b)),0x90 ;
>     142: .pushsection .altinstructions,"a" ;
> 	altinstr_entry 140b,143f,\ft_flags2,142b-140b,144f-143f ;
> 	.popsection ;
> 	.pushsection .altinstr_replacement,"ax" ;
>     143: \newinstr2 ;
>     144: .popsection ;
>    .endm
>
> The only label that is ambiguous is 140, however they all reference
> the same spot, so that doesn't matter.
>
> NOTE: obviously only @oldinstr may be an alternative; making @newinstr
> an alternative would mean patching .altinstr_replacement which very
> likely isn't what is intended, also the labels will be confused in
> that case.

Reviewed-by: Nikolay Borisov <nik.borisov@xxxxxxxx>

Ps. I feel very "enlightened" knowing that GAS uses -1 to represent true 
...