linux-kernel - [PATCH v2] x86/paravirt: Disable virt spinlock on bare metal

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Date: Sun, 26 May 2024 09:58:47 +0800
From: Chen Yu <yu.c.chen@...el.com>
To: Dave Hansen <dave.hansen@...ux.intel.com>,
	Juergen Gross <jgross@...e.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Ajay Kaher <ajay.kaher@...adcom.com>
Cc: x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Arnd Bergmann <arnd@...db.de>,
	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 v2] x86/paravirt: Disable virt spinlock on bare metal

The kernel can change spinlock behavior when running as a guest. But
this guest-friendly behavior causes performance problems on bare metal.
So there's a 'virt_spin_lock_key' static key to switch between the two
modes.

The static key is always enabled by default (run in guest mode) and
should be disabled for bare metal (and in some guests that want native
behavior).

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 incorrectly 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 it is on bare metal,
regardless of CONFIG_PARAVIRT_SPINLOCKS.

Fixes: ce0a1b608bfc ("x86/paravirt: Silence unused native_pv_lock_init() function warning")
Suggested-by: Dave Hansen <dave.hansen@...ux.intel.com>
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>
Reviewed-by: Juergen Gross <jgross@...e.com>
Signed-off-by: Chen Yu <yu.c.chen@...el.com>
---
v1->v2:
  Refine the commit log per Dave's suggestion.
  Simplify the fix by directly disabling the virt_spin_lock_key on bare metal.
  Collect Reviewed-by from Juergen.
---
 arch/x86/kernel/paravirt.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
index 5358d43886ad..c193c9e60a1b 100644
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -55,8 +55,7 @@ DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);

 void __init native_pv_lock_init(void)
 {
-	if (IS_ENABLED(CONFIG_PARAVIRT_SPINLOCKS) &&
-	    !boot_cpu_has(X86_FEATURE_HYPERVISOR))
+	if (!boot_cpu_has(X86_FEATURE_HYPERVISOR))
 		static_branch_disable(&virt_spin_lock_key);
 }

-- 
2.25.1