linux-kernel - [PATCH] x86/paravirt: Disable virt spinlock when CONFIG_PARAVIRT

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Date: Thu, 16 May 2024 21:02:44 +0800
From: Chen Yu <yu.c.chen@...el.com>
To: Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Juergen Gross <jgross@...e.com>,
	Ajay Kaher <ajay.kaher@...adcom.com>,
	Alexey Makhalov <alexey.amakhalov@...adcom.com>,
	Arnd Bergmann <arnd@...db.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	x86@...nel.org,
	virtualization@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	Chen Yu <yu.c.chen@...el.com>,
	Qiuxu Zhuo <qiuxu.zhuo@...el.com>,
	Prem Nath Dey <prem.nath.dey@...el.com>,
	Xiaoping Zhou <xiaoping.zhou@...el.com>
Subject: [PATCH] x86/paravirt: Disable virt spinlock when CONFIG_PARAVIRT_SPINLOCKS disabled

Performance drop is reported when running encode/decode workload and
BenchSEE cache sub-workload.
Bisect points to commit ce0a1b608bfc ("x86/paravirt: Silence unused
native_pv_lock_init() function warning"). When CONFIG_PARAVIRT_SPINLOCKS
is disabled the virt_spin_lock_key is set to true on bare-metal.
The qspinlock degenerates to test-and-set spinlock, which decrease the
performance on bare-metal.

Fix this by disabling virt_spin_lock_key if CONFIG_PARAVIRT_SPINLOCKS
is not set, or it is on bare-metal.

Fixes: ce0a1b608bfc ("x86/paravirt: Silence unused native_pv_lock_init() function warning")
Suggested-by: Qiuxu Zhuo <qiuxu.zhuo@...el.com>
Reported-by: Prem Nath Dey <prem.nath.dey@...el.com>
Reported-by: Xiaoping Zhou <xiaoping.zhou@...el.com>
Signed-off-by: Chen Yu <yu.c.chen@...el.com>
---
 arch/x86/kernel/paravirt.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
index 5358d43886ad..ee51c0949ed8 100644
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -55,7 +55,7 @@ DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);
 
 void __init native_pv_lock_init(void)
 {
-	if (IS_ENABLED(CONFIG_PARAVIRT_SPINLOCKS) &&
+	if (!IS_ENABLED(CONFIG_PARAVIRT_SPINLOCKS) ||
 	    !boot_cpu_has(X86_FEATURE_HYPERVISOR))
 		static_branch_disable(&virt_spin_lock_key);
 }
-- 
2.25.1