linux-kernel - Re: [PATCH 02/14] x86/alternatives: Add nested alternatives macros

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Date: Fri, 31 May 2024 20:09:08 +0200
From: Borislav Petkov <bp@...en8.de>
To: Uros Bizjak <ubizjak@...il.com>
Cc: Borislav Petkov <bp@...nel.org>, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 02/14] x86/alternatives: Add nested alternatives macros

On Fri, May 31, 2024 at 05:10:08PM +0200, Uros Bizjak wrote:
> The intention of '%P' operand modifier is primarily to be used with PIC,
> where ...

Ok, I'll fold in the following. I'd like to have this documented
somewhere in the tree too.

Thx.

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index f02867092a49..4973edbe9289 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -236,9 +236,18 @@ static inline int alternatives_text_reserved(void *start, void *end)
 	asm_inline volatile(ALTERNATIVE(oldinstr, newinstr, ft_flags)	\
 		: output : "i" (0), ## input)
 
-/* Like alternative_io, but for replacing a direct call with another one. */
+/*
+ * Like alternative_io, but for replacing a direct call with another one.
+ *
+ * Use the %c operand modifier which is the generic way to print a bare
+ * constant expression with all syntax-specific punctuation omitted. %P
+ * is the x86-specific variant which can handle constants too, for
+ * historical reasons, but it should be used primarily for PIC
+ * references: i.e., if used for a function, it would add the PLT
+ * suffix.
+ */
 #define alternative_call(oldfunc, newfunc, ft_flags, output, input...)	\
-	asm_inline volatile(ALTERNATIVE("call %P[old]", "call %P[new]", ft_flags) \
+	asm_inline volatile(ALTERNATIVE("call %c[old]", "call %c[new]", ft_flags) \
 		: output : [old] "i" (oldfunc), [new] "i" (newfunc), ## input)
 
 /*
@@ -249,8 +258,8 @@ static inline int alternatives_text_reserved(void *start, void *end)
  */
 #define alternative_call_2(oldfunc, newfunc1, ft_flags1, newfunc2, ft_flags2,		\
 			   output, input...)						\
-	asm_inline volatile(ALTERNATIVE_2("call %P[old]", "call %P[new1]", ft_flags1,	\
-		"call %P[new2]", ft_flags2)						\
+	asm_inline volatile(ALTERNATIVE_2("call %c[old]", "call %c[new1]", ft_flags1,	\
+		"call %c[new2]", ft_flags2)						\
 		: output, ASM_CALL_CONSTRAINT						\
 		: [old] "i" (oldfunc), [new1] "i" (newfunc1),				\
 		  [new2] "i" (newfunc2), ## input)

-- 
Regards/Gruss,
    Boris.

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