linux-kernel - [PATCH v2] x86/ioperm: Use atomic64_inc_return() in ksys

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Message-ID: <20241024182833.177994-1-ubizjak@gmail.com>
Date: Thu, 24 Oct 2024 20:28:14 +0200
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 v2] x86/ioperm: Use atomic64_inc_return() in ksys_ioperm()

Use atomic64_inc_return(&ref) instead of atomic64_add_return(1, &ref)
to use optimized implementation on targets that define
atomic_inc_return() and to remove now unneeded initialization of
%eax/%edx register pair before the call toatomic64_inc_return().

On x86_32 the code improves from:

 1b0:    b9 00 00 00 00           mov    $0x0,%ecx
            1b1: R_386_32    .bss
 1b5:    89 43 0c                 mov    %eax,0xc(%ebx)
 1b8:    31 d2                    xor    %edx,%edx
 1ba:    b8 01 00 00 00           mov    $0x1,%eax
 1bf:    e8 fc ff ff ff           call   1c0 <ksys_ioperm+0xa8>
            1c0: R_386_PC32    atomic64_add_return_cx8
 1c4:    89 03                    mov    %eax,(%ebx)
 1c6:    89 53 04                 mov    %edx,0x4(%ebx)

to:

 1b0:    be 00 00 00 00           mov    $0x0,%esi
            1b1: R_386_32    .bss
 1b5:    89 43 0c                 mov    %eax,0xc(%ebx)
 1b8:    e8 fc ff ff ff           call   1b9 <ksys_ioperm+0xa1>
            1b9: R_386_PC32    atomic64_inc_return_cx8
 1bd:    89 03                    mov    %eax,(%ebx)
 1bf:    89 53 04                 mov    %edx,0x4(%ebx)

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>
---
v2: Mention specific code improvement on x86_32 target instead
    of register pressure issue
---
 arch/x86/kernel/ioport.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/ioport.c b/arch/x86/kernel/ioport.c
index e2fab3ceb09f..6290dd120f5e 100644
--- a/arch/x86/kernel/ioport.c
+++ b/arch/x86/kernel/ioport.c
@@ -144,7 +144,7 @@ long ksys_ioperm(unsigned long from, unsigned long num, int turn_on)
 	 * Update the sequence number to force a TSS update on return to
 	 * user mode.
 	 */
-	iobm->sequence = atomic64_add_return(1, &io_bitmap_sequence);
+	iobm->sequence = atomic64_inc_return(&io_bitmap_sequence);
 
 	return 0;
 }
-- 
2.42.0