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Message-ID: <20231001104053.9644-2-ubizjak@gmail.com>
Date: Sun, 1 Oct 2023 12:39:10 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: linux-kernel@...r.kernel.org, x86@...nel.org,
xen-devel@...ts.xenproject.org
Cc: Uros Bizjak <ubizjak@...il.com>,
Peter Zijlstra <peterz@...radead.org>,
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 2/3] locking/x86: Introduce arch_sync_try_cmpxchg
Introduce arch_sync_try_cmpxchg macro to improve code using
sync_try_cmpxchg locking primitive. The new definitions use existing
__raw_try_cmpxchg macros, but use its own "lock; " prefix.
The new macros improve assembly of the cmpxchg loop in
evtchn_fifo_unmask() from drivers/xen/events/events_fifo.c from:
57a: 85 c0 test %eax,%eax
57c: 78 52 js 5d0 <...>
57e: 89 c1 mov %eax,%ecx
580: 25 ff ff ff af and $0xafffffff,%eax
585: c7 04 24 00 00 00 00 movl $0x0,(%rsp)
58c: 81 e1 ff ff ff ef and $0xefffffff,%ecx
592: 89 4c 24 04 mov %ecx,0x4(%rsp)
596: 89 44 24 08 mov %eax,0x8(%rsp)
59a: 8b 74 24 08 mov 0x8(%rsp),%esi
59e: 8b 44 24 04 mov 0x4(%rsp),%eax
5a2: f0 0f b1 32 lock cmpxchg %esi,(%rdx)
5a6: 89 04 24 mov %eax,(%rsp)
5a9: 8b 04 24 mov (%rsp),%eax
5ac: 39 c1 cmp %eax,%ecx
5ae: 74 07 je 5b7 <...>
5b0: a9 00 00 00 40 test $0x40000000,%eax
5b5: 75 c3 jne 57a <...>
<...>
to:
578: a9 00 00 00 40 test $0x40000000,%eax
57d: 74 2b je 5aa <...>
57f: 85 c0 test %eax,%eax
581: 78 40 js 5c3 <...>
583: 89 c1 mov %eax,%ecx
585: 25 ff ff ff af and $0xafffffff,%eax
58a: 81 e1 ff ff ff ef and $0xefffffff,%ecx
590: 89 4c 24 04 mov %ecx,0x4(%rsp)
594: 89 44 24 08 mov %eax,0x8(%rsp)
598: 8b 4c 24 08 mov 0x8(%rsp),%ecx
59c: 8b 44 24 04 mov 0x4(%rsp),%eax
5a0: f0 0f b1 0a lock cmpxchg %ecx,(%rdx)
5a4: 89 44 24 04 mov %eax,0x4(%rsp)
5a8: 75 30 jne 5da <...>
<...>
5da: 8b 44 24 04 mov 0x4(%rsp),%eax
5de: eb 98 jmp 578 <...>
The new code removes move instructions from 585: 5a6: and 5a9:
and the compare from 5ac:. Additionally, the compiler assumes that
cmpxchg success is more probable and optimizes code flow accordingly.
Cc: Peter Zijlstra <peterz@...radead.org>
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>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
v2: Improve commit description.
---
arch/x86/include/asm/cmpxchg.h | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/arch/x86/include/asm/cmpxchg.h b/arch/x86/include/asm/cmpxchg.h
index d53636506134..5612648b0202 100644
--- a/arch/x86/include/asm/cmpxchg.h
+++ b/arch/x86/include/asm/cmpxchg.h
@@ -221,12 +221,18 @@ extern void __add_wrong_size(void)
#define __try_cmpxchg(ptr, pold, new, size) \
__raw_try_cmpxchg((ptr), (pold), (new), (size), LOCK_PREFIX)
+#define __sync_try_cmpxchg(ptr, pold, new, size) \
+ __raw_try_cmpxchg((ptr), (pold), (new), (size), "lock; ")
+
#define __try_cmpxchg_local(ptr, pold, new, size) \
__raw_try_cmpxchg((ptr), (pold), (new), (size), "")
#define arch_try_cmpxchg(ptr, pold, new) \
__try_cmpxchg((ptr), (pold), (new), sizeof(*(ptr)))
+#define arch_sync_try_cmpxchg(ptr, pold, new) \
+ __sync_try_cmpxchg((ptr), (pold), (new), sizeof(*(ptr)))
+
#define arch_try_cmpxchg_local(ptr, pold, new) \
__try_cmpxchg_local((ptr), (pold), (new), sizeof(*(ptr)))
--
2.41.0
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