linux-kernel - [PATCH] x86/asm: Use asm_inline() instead of asm() in __untagged

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Message-ID: <20250314093111.654359-1-ubizjak@gmail.com>
Date: Fri, 14 Mar 2025 10:30:55 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: x86@...nel.org,
	linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH] x86/asm: Use asm_inline() instead of asm() in __untagged_addr()

Use asm_inline() to instruct the compiler that the size of asm()
is the minimum size of one instruction, ignoring how many instructions
the compiler thinks it is. ALTERNATIVE macro that expands to several
pseudo directives causes instruction length estimate to count
more than 20 instructions.

bloat-o-meter reports minimal code size increase
(x86_64 defconfig with CONFIG_ADDRESS_MASKING, gcc-14.2.1):

  add/remove: 2/2 grow/shrink: 5/1 up/down: 2365/-1995 (370)

	Function                          old     new   delta
	-----------------------------------------------------
	do_get_mempolicy                    -    1449   +1449
	copy_nodes_to_user                  -     226    +226
	__x64_sys_get_mempolicy            35     213    +178
	syscall_user_dispatch_set_config  157     332    +175
	__ia32_sys_get_mempolicy           31     206    +175
	set_syscall_user_dispatch          29     181    +152
	__do_sys_mremap                  2073    2083     +10
	sp_insert                         133     117     -16
	task_set_syscall_user_dispatch    172       -    -172
	kernel_get_mempolicy             1807       -   -1807

  Total: Before=21423151, After=21423521, chg +0.00%

The code size increase is due to the compiler inlining
more functions that inline untagged_addr().

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
---
 arch/x86/include/asm/uaccess_64.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
index c52f0133425b..3c1bec3a0405 100644
--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -26,8 +26,8 @@ extern unsigned long USER_PTR_MAX;
  */
 static inline unsigned long __untagged_addr(unsigned long addr)
 {
-	asm (ALTERNATIVE("",
-			 "and " __percpu_arg([mask]) ", %[addr]", X86_FEATURE_LAM)
+	asm_inline (ALTERNATIVE("", "and " __percpu_arg([mask]) ", %[addr]",
+				X86_FEATURE_LAM)
 	     : [addr] "+r" (addr)
 	     : [mask] "m" (__my_cpu_var(tlbstate_untag_mask)));
 
-- 
2.48.1