Linux Virtualization - Sep 2017 - [PATCH v2 0/2] guard virt_spin_lock() with a static key

With virt_spin_lock() being guarded by a static key the bare metal case
can be optimized by patching the call away completely. In case a kernel
running as a guest it can decide whether to use paravitualized
spinlocks, the current fallback to the unfair test-and-set scheme, or
to mimic the bare metal behavior.

V2:
- use static key instead of making virt_spin_lock() a pvops function

Juergen Gross (2):
  paravirt/locks: use new static key for controlling call of
    virt_spin_lock()
  paravirt,xen: correct xen_nopvspin case

 arch/x86/include/asm/qspinlock.h     | 11 +++++++++++
 arch/x86/kernel/paravirt-spinlocks.c |  6 ++++++
 arch/x86/kernel/smpboot.c            |  2 ++
 arch/x86/xen/spinlock.c              |  2 ++
 kernel/locking/qspinlock.c           |  4 ++++
 5 files changed, 25 insertions(+)

-- 
2.12.3

There are cases where a guest tries to switch spinlocks to bare metal
behavior (e.g. by setting "xen_nopvspin" boot parameter). Today this
has the downside of falling back to unfair test and set scheme for
qspinlocks due to virt_spin_lock() detecting the virtualized
environment.

Add a static key controlling whether virt_spin_lock() should be
called or not. When running on bare metal set the new key to false.

Signed-off-by: Juergen Gross <jgross at suse.com>
---
 arch/x86/include/asm/qspinlock.h     | 11 +++++++++++
 arch/x86/kernel/paravirt-spinlocks.c |  6 ++++++
 arch/x86/kernel/smpboot.c            |  2 ++
 kernel/locking/qspinlock.c           |  4 ++++
 4 files changed, 23 insertions(+)

diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
index 48a706f641f2..fc39389f196b 100644
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -1,6 +1,7 @@
 #ifndef _ASM_X86_QSPINLOCK_H
 #define _ASM_X86_QSPINLOCK_H
 
+#include <linux/jump_label.h>
 #include <asm/cpufeature.h>
 #include <asm-generic/qspinlock_types.h>
 #include <asm/paravirt.h>
@@ -46,9 +47,15 @@ static inline void queued_spin_unlock(struct qspinlock *lock)
 #endif
 
 #ifdef CONFIG_PARAVIRT
+DECLARE_STATIC_KEY_TRUE(virt_spin_lock_key);
+
+void native_pv_lock_init(void) __init;
+
 #define virt_spin_lock virt_spin_lock
 static inline bool virt_spin_lock(struct qspinlock *lock)
 {
+	if (!static_branch_likely(&virt_spin_lock_key))
+		return false;
 	if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
 		return false;
 
@@ -65,6 +72,10 @@ static inline bool virt_spin_lock(struct qspinlock *lock)
 
 	return true;
 }
+#else
+static inline void native_pv_lock_init(void)
+{
+}
 #endif /* CONFIG_PARAVIRT */
 
 #include <asm-generic/qspinlock.h>
diff --git a/arch/x86/kernel/paravirt-spinlocks.c
b/arch/x86/kernel/paravirt-spinlocks.c
index 8f2d1c9d43a8..2fc65ddea40d 100644
--- a/arch/x86/kernel/paravirt-spinlocks.c
+++ b/arch/x86/kernel/paravirt-spinlocks.c
@@ -42,3 +42,9 @@ struct pv_lock_ops pv_lock_ops = {
 #endif /* SMP */
 };
 EXPORT_SYMBOL(pv_lock_ops);
+
+void __init native_pv_lock_init(void)
+{
+	if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
+		static_branch_disable(&virt_spin_lock_key);
+}
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 54b9e89d4d6b..21500d3ba359 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -77,6 +77,7 @@
 #include <asm/i8259.h>
 #include <asm/realmode.h>
 #include <asm/misc.h>
+#include <asm/qspinlock.h>
 
 /* Number of siblings per CPU package */
 int smp_num_siblings = 1;
@@ -1381,6 +1382,7 @@ void __init native_smp_prepare_boot_cpu(void)
 	/* already set me in cpu_online_mask in boot_cpu_init() */
 	cpumask_set_cpu(me, cpu_callout_mask);
 	cpu_set_state_online(me);
+	native_pv_lock_init();
 }
 
 void __init native_smp_cpus_done(unsigned int max_cpus)
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 294294c71ba4..838d235b87ef 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -76,6 +76,10 @@
 #define MAX_NODES	4
 #endif
 
+#ifdef CONFIG_PARAVIRT
+DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);
+#endif
+
 /*
  * Per-CPU queue node structures; we can never have more than 4 nested
  * contexts: task, softirq, hardirq, nmi.
-- 
2.12.3

With the boot parameter "xen_nopvspin" specified a Xen guest should
not
make use of paravirt spinlocks, but behave as if running on bare
metal. This is not true, however, as the qspinlock code will fall back
to a test-and-set scheme when it is detecting a hypervisor.

In order to avoid this disable the virt_spin_lock_key.

Signed-off-by: Juergen Gross <jgross at suse.com>
---
 arch/x86/xen/spinlock.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 25a7c4302ce7..e8ab80ad7a6f 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -10,6 +10,7 @@
 #include <linux/slab.h>
 
 #include <asm/paravirt.h>
+#include <asm/qspinlock.h>
 
 #include <xen/interface/xen.h>
 #include <xen/events.h>
@@ -129,6 +130,7 @@ void __init xen_init_spinlocks(void)
 
 	if (!xen_pvspin) {
 		printk(KERN_DEBUG "xen: PV spinlocks disabled\n");
+		static_branch_disable(&virt_spin_lock_key);
 		return;
 	}
 	printk(KERN_DEBUG "xen: PV spinlocks enabled\n");
-- 
2.12.3

On 09/06/2017 11:29 AM, Juergen Gross wrote:
> There are cases where a guest tries to switch spinlocks to bare metal
> behavior (e.g. by setting "xen_nopvspin" boot parameter). Today
this
> has the downside of falling back to unfair test and set scheme for
> qspinlocks due to virt_spin_lock() detecting the virtualized
> environment.
>
> Add a static key controlling whether virt_spin_lock() should be
> called or not. When running on bare metal set the new key to false.
>
> Signed-off-by: Juergen Gross <jgross at suse.com>
> ---
>  arch/x86/include/asm/qspinlock.h     | 11 +++++++++++
>  arch/x86/kernel/paravirt-spinlocks.c |  6 ++++++
>  arch/x86/kernel/smpboot.c            |  2 ++
>  kernel/locking/qspinlock.c           |  4 ++++
>  4 files changed, 23 insertions(+)
>
> diff --git a/arch/x86/include/asm/qspinlock.h
b/arch/x86/include/asm/qspinlock.h
> index 48a706f641f2..fc39389f196b 100644
> --- a/arch/x86/include/asm/qspinlock.h
> +++ b/arch/x86/include/asm/qspinlock.h
> @@ -1,6 +1,7 @@
>  #ifndef _ASM_X86_QSPINLOCK_H
>  #define _ASM_X86_QSPINLOCK_H
>  
> +#include <linux/jump_label.h>
>  #include <asm/cpufeature.h>
>  #include <asm-generic/qspinlock_types.h>
>  #include <asm/paravirt.h>
> @@ -46,9 +47,15 @@ static inline void queued_spin_unlock(struct qspinlock
*lock)
>  #endif
>  
>  #ifdef CONFIG_PARAVIRT
> +DECLARE_STATIC_KEY_TRUE(virt_spin_lock_key);
> +
> +void native_pv_lock_init(void) __init;
> +
>  #define virt_spin_lock virt_spin_lock
>  static inline bool virt_spin_lock(struct qspinlock *lock)
>  {
> +	if (!static_branch_likely(&virt_spin_lock_key))
> +		return false;
>  	if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
>  		return false;
>  
> @@ -65,6 +72,10 @@ static inline bool virt_spin_lock(struct qspinlock
*lock)
>  
>  	return true;
>  }
> +#else
> +static inline void native_pv_lock_init(void)
> +{
> +}
>  #endif /* CONFIG_PARAVIRT */
>  
>  #include <asm-generic/qspinlock.h>
> diff --git a/arch/x86/kernel/paravirt-spinlocks.c
b/arch/x86/kernel/paravirt-spinlocks.c
> index 8f2d1c9d43a8..2fc65ddea40d 100644
> --- a/arch/x86/kernel/paravirt-spinlocks.c
> +++ b/arch/x86/kernel/paravirt-spinlocks.c
> @@ -42,3 +42,9 @@ struct pv_lock_ops pv_lock_ops = {
>  #endif /* SMP */
>  };
>  EXPORT_SYMBOL(pv_lock_ops);
> +
> +void __init native_pv_lock_init(void)
> +{
> +	if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
> +		static_branch_disable(&virt_spin_lock_key);
> +}
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 54b9e89d4d6b..21500d3ba359 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -77,6 +77,7 @@
>  #include <asm/i8259.h>
>  #include <asm/realmode.h>
>  #include <asm/misc.h>
> +#include <asm/qspinlock.h>
>  
>  /* Number of siblings per CPU package */
>  int smp_num_siblings = 1;
> @@ -1381,6 +1382,7 @@ void __init native_smp_prepare_boot_cpu(void)
>  	/* already set me in cpu_online_mask in boot_cpu_init() */
>  	cpumask_set_cpu(me, cpu_callout_mask);
>  	cpu_set_state_online(me);
> +	native_pv_lock_init();
>  }
>  
>  void __init native_smp_cpus_done(unsigned int max_cpus)
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 294294c71ba4..838d235b87ef 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -76,6 +76,10 @@
>  #define MAX_NODES	4
>  #endif
>  
> +#ifdef CONFIG_PARAVIRT
> +DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);
> +#endif
> +
>  /*
>   * Per-CPU queue node structures; we can never have more than 4 nested
>   * contexts: task, softirq, hardirq, nmi.
Acked-by: Waiman Long <longman at redhat.com>

On 09/06/2017 11:29 AM, Juergen Gross wrote:
> With the boot parameter "xen_nopvspin" specified a Xen guest
should not
> make use of paravirt spinlocks, but behave as if running on bare
> metal. This is not true, however, as the qspinlock code will fall back
> to a test-and-set scheme when it is detecting a hypervisor.
>
> In order to avoid this disable the virt_spin_lock_key.
>
> Signed-off-by: Juergen Gross <jgross at suse.com>
> ---
>  arch/x86/xen/spinlock.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
> index 25a7c4302ce7..e8ab80ad7a6f 100644
> --- a/arch/x86/xen/spinlock.c
> +++ b/arch/x86/xen/spinlock.c
> @@ -10,6 +10,7 @@
>  #include <linux/slab.h>
>  
>  #include <asm/paravirt.h>
> +#include <asm/qspinlock.h>
>  
>  #include <xen/interface/xen.h>
>  #include <xen/events.h>
> @@ -129,6 +130,7 @@ void __init xen_init_spinlocks(void)
>  
>  	if (!xen_pvspin) {
>  		printk(KERN_DEBUG "xen: PV spinlocks disabled\n");
> +		static_branch_disable(&virt_spin_lock_key);
>  		return;
>  	}
>  	printk(KERN_DEBUG "xen: PV spinlocks enabled\n");
Acked-by: Waiman Long <longman at redhat.com>