linux-kernel - RE: [PATCH v2 06/14] x86/asm: Fix register order

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Date:   Fri, 22 Oct 2021 19:27:06 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alexei.starovoitov@...il.com" <alexei.starovoitov@...il.com>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>
Subject: RE: [PATCH v2 06/14] x86/asm: Fix register order

From: Peter Zijlstra
> Sent: 20 October 2021 11:45
> 
> Ensure the register order is correct; this allows for easy translation
> between register number and trampoline and vice-versa.

There is an alternate cpp pattern that can be used to have the
same effect.
Possibly less error prone.
Stealing some text from 2 patches you'd have:

asm/GEN-for-each-reg.h would contain:

#define GEN_FOR_EACH_REG(GEN) \
  GEN(rax) \
  GEN(rcx) \
  GEN(rdx) \
  GEN(rbx) \
  GEN(rsp) \
  GEN(rbp) \
  GEN(rsi) \
  GEN(rdi) \
  GEN(r8)  \
  GEN(r9)  \
  GEN(r10)

and the user would contain:

#include <asm/GEN-for-each-reg.h>

#define GEN(reg) \
 	extern asmlinkage void __x86_indirect_thunk_ ## reg (void);
GEN_FOR_EACH_REG(GEN)
#undef GEN

#define GEN(reg) \
 	extern asmlinkage void __x86_indirect_alt_call_ ## reg (void);
GEN_FOR_EACH_REG(GEN)
#undef GEN

It is probably possible to use ... and _VA_LIST_.
Something like:

#define GEN_FOR_EACH_REG(GEN, ...) \
  GEN(rax, _VA_LIST_) \
  ...

#define GEN_THUNK(reg, thunk) \
	extern asmlinkage void thunk ## reg (void);

GEN_FOR_EACH_REG(GEN_THUNK, __x86_indirect_thunk_)
GEN_FOR_EACH_REG(GEN_THUNK, __x86_indirect_alt_call_)
#undef GEN_THUNK

	David

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