linux-kernel - [PATCH] locking/x86/xen: Use try_cmpxchg() in xen_alloc_p2m

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Message-ID: <20240405083335.507471-1-ubizjak@gmail.com>
Date: Fri,  5 Apr 2024 10:32:53 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: xen-devel@...ts.xenproject.org,
	x86@...nel.org,
	linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	Juergen Gross <jgross@...e.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.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] locking/x86/xen: Use try_cmpxchg() in xen_alloc_p2m_entry()

Use try_cmpxchg() instead of cmpxchg(*ptr, old, new) == old.

The x86 CMPXCHG instruction returns success in the ZF flag,
so this change saves a compare after CMPXCHG.

Also, try_cmpxchg() implicitly assigns old *ptr value to "old"
when CMPXCHG fails. There is no need to explicitly assign
old *ptr value to the temporary, which can simplify the
surrounding source code.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Juergen Gross <jgross@...e.com>
Cc: Boris Ostrovsky <boris.ostrovsky@...cle.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/xen/p2m.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index 9bdc3b656b2c..99918beccd80 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -555,7 +555,6 @@ int xen_alloc_p2m_entry(unsigned long pfn)
 			/* Separately check the mid mfn level */
 			unsigned long missing_mfn;
 			unsigned long mid_mfn_mfn;
-			unsigned long old_mfn;
 
 			mid_mfn = alloc_p2m_page();
 			if (!mid_mfn)
@@ -565,12 +564,12 @@ int xen_alloc_p2m_entry(unsigned long pfn)
 
 			missing_mfn = virt_to_mfn(p2m_mid_missing_mfn);
 			mid_mfn_mfn = virt_to_mfn(mid_mfn);
-			old_mfn = cmpxchg(top_mfn_p, missing_mfn, mid_mfn_mfn);
-			if (old_mfn != missing_mfn) {
-				free_p2m_page(mid_mfn);
-				mid_mfn = mfn_to_virt(old_mfn);
-			} else {
+			/* try_cmpxchg() updates missing_mfn on failure. */
+			if (try_cmpxchg(top_mfn_p, &missing_mfn, mid_mfn_mfn)) {
 				p2m_top_mfn_p[topidx] = mid_mfn;
+			} else {
+				free_p2m_page(mid_mfn);
+				mid_mfn = mfn_to_virt(missing_mfn);
 			}
 		}
 	} else {
-- 
2.44.0