Re: [Xen-devel] [PATCH 3/4] paravirt: add virt_spin_lock pvops function

[Juergen Gross <jgross@xxxxxxxx>]

On 05/09/17 16:10, Waiman Long wrote:
> On 09/05/2017 09:24 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.
>>
>> Make virt_spin_lock() a paravirt operation in order to enable users
>> to select an explicit behavior like bare metal.
>>
>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>> ---
>>  arch/x86/include/asm/paravirt.h       |  5 ++++
>>  arch/x86/include/asm/paravirt_types.h |  1 +
>>  arch/x86/include/asm/qspinlock.h      | 48 
>> ++++++++++++++++++++++++-----------
>>  arch/x86/kernel/paravirt-spinlocks.c  | 14 ++++++++++
>>  arch/x86/kernel/smpboot.c             |  2 ++
>>  5 files changed, 55 insertions(+), 15 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/paravirt.h 
>> b/arch/x86/include/asm/paravirt.h
>> index c25dd22f7c70..d9e954fb37df 100644
>> --- a/arch/x86/include/asm/paravirt.h
>> +++ b/arch/x86/include/asm/paravirt.h
>> @@ -725,6 +725,11 @@ static __always_inline bool pv_vcpu_is_preempted(long 
>> cpu)
>>      return PVOP_CALLEE1(bool, pv_lock_ops.vcpu_is_preempted, cpu);
>>  }
>>  
>> +static __always_inline bool pv_virt_spin_lock(struct qspinlock *lock)
>> +{
>> +    return PVOP_CALLEE1(bool, pv_lock_ops.virt_spin_lock, lock);
>> +}
>> +
>>  #endif /* SMP && PARAVIRT_SPINLOCKS */
>>  
>>  #ifdef CONFIG_X86_32
>> diff --git a/arch/x86/include/asm/paravirt_types.h 
>> b/arch/x86/include/asm/paravirt_types.h
>> index 19efefc0e27e..928f5e7953a7 100644
>> --- a/arch/x86/include/asm/paravirt_types.h
>> +++ b/arch/x86/include/asm/paravirt_types.h
>> @@ -319,6 +319,7 @@ struct pv_lock_ops {
>>      void (*kick)(int cpu);
>>  
>>      struct paravirt_callee_save vcpu_is_preempted;
>> +    struct paravirt_callee_save virt_spin_lock;
>>  } __no_randomize_layout;
>>  
>>  /* This contains all the paravirt structures: we get a convenient
>> diff --git a/arch/x86/include/asm/qspinlock.h 
>> b/arch/x86/include/asm/qspinlock.h
>> index 48a706f641f2..fbd98896385c 100644
>> --- a/arch/x86/include/asm/qspinlock.h
>> +++ b/arch/x86/include/asm/qspinlock.h
>> @@ -17,6 +17,25 @@ static inline void native_queued_spin_unlock(struct 
>> qspinlock *lock)
>>      smp_store_release((u8 *)lock, 0);
>>  }
>>  
>> +static inline bool native_virt_spin_lock(struct qspinlock *lock)
>> +{
>> +    if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
>> +            return false;
>> +
>> +    /*
>> +     * On hypervisors without PARAVIRT_SPINLOCKS support we fall
>> +     * back to a Test-and-Set spinlock, because fair locks have
>> +     * horrible lock 'holder' preemption issues.
>> +     */
>> +
>> +    do {
>> +            while (atomic_read(&lock->val) != 0)
>> +                    cpu_relax();
>> +    } while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0);
>> +
>> +    return true;
>> +}
>> +
>>  #ifdef CONFIG_PARAVIRT_SPINLOCKS
>>  extern void native_queued_spin_lock_slowpath(struct qspinlock *lock, u32 
>> val);
>>  extern void __pv_init_lock_hash(void);
>> @@ -38,33 +57,32 @@ static inline bool vcpu_is_preempted(long cpu)
>>  {
>>      return pv_vcpu_is_preempted(cpu);
>>  }
>> +
>> +void native_pv_lock_init(void) __init;
>>  #else
>>  static inline void queued_spin_unlock(struct qspinlock *lock)
>>  {
>>      native_queued_spin_unlock(lock);
>>  }
>> +
>> +static inline void native_pv_lock_init(void)
>> +{
>> +}
>>  #endif
>>  
>>  #ifdef CONFIG_PARAVIRT
>>  #define virt_spin_lock virt_spin_lock
>> +#ifdef CONFIG_PARAVIRT_SPINLOCKS
>>  static inline bool virt_spin_lock(struct qspinlock *lock)
>>  {
>> -    if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
>> -            return false;
> 
> Have you consider just add one more jump label here to skip
> virt_spin_lock when KVM or Xen want to do so?

Why? Did you look at patch 4? This is the way to do it...

> 
>> -
>> -    /*
>> -     * On hypervisors without PARAVIRT_SPINLOCKS support we fall
>> -     * back to a Test-and-Set spinlock, because fair locks have
>> -     * horrible lock 'holder' preemption issues.
>> -     */
>> -
>> -    do {
>> -            while (atomic_read(&lock->val) != 0)
>> -                    cpu_relax();
>> -    } while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0);
>> -
>> -    return true;
>> +    return pv_virt_spin_lock(lock);
>> +}
>> +#else
>> +static inline bool virt_spin_lock(struct qspinlock *lock)
>> +{
>> +    return native_virt_spin_lock(lock);
>>  }
>> +#endif /* CONFIG_PARAVIRT_SPINLOCKS */
>>  #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 26e4bd92f309..1be187ef8a38 100644
>> --- a/arch/x86/kernel/paravirt-spinlocks.c
>> +++ b/arch/x86/kernel/paravirt-spinlocks.c
>> @@ -20,6 +20,12 @@ bool pv_is_native_spin_unlock(void)
>>              __raw_callee_save___native_queued_spin_unlock;
>>  }
>>  
>> +__visible bool __native_virt_spin_lock(struct qspinlock *lock)
>> +{
>> +    return native_virt_spin_lock(lock);
>> +}
>> +PV_CALLEE_SAVE_REGS_THUNK(__native_virt_spin_lock);
> 
> I have some concern about the overhead of register saving/restoring have
> on spin lock performance in case the kernel is under a non-KVM/Xen
> hypervisor.

We are on the slow path already.


Juergen

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