[v3,1/4] x86/kvm: Add "nopvspin" parameter to disable PV spinlocks
diff mbox series

Message ID 1569847479-13201-2-git-send-email-zhenzhong.duan@oracle.com
State New
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Series
  • Add a unified parameter "nopvspin"
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Commit Message

Zhenzhong Duan Sept. 30, 2019, 12:44 p.m. UTC
There are cases where a guest tries to switch spinlocks to bare metal
behavior (e.g. by setting "xen_nopvspin" on XEN platform and
"hv_nopvspin" on HYPER_V).

That feature is missed on KVM, add a new parameter "nopvspin" to disable
PV spinlocks for KVM guest.

This new parameter is also used to replace "xen_nopvspin" and
"hv_nopvspin".

The global variable pvspin isn't defined as __initdata as it's used at
runtime by XEN guest.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krcmar <rkrcmar@redhat.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Wanpeng Li <wanpengli@tencent.com>
Cc: Jim Mattson <jmattson@google.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
---
 Documentation/admin-guide/kernel-parameters.txt | 4 ++++
 arch/x86/include/asm/qspinlock.h                | 1 +
 arch/x86/kernel/kvm.c                           | 7 +++++++
 kernel/locking/qspinlock.c                      | 7 +++++++
 4 files changed, 19 insertions(+)

Comments

Sean Christopherson Oct. 2, 2019, 5:10 p.m. UTC | #1
On Mon, Sep 30, 2019 at 08:44:36PM +0800, Zhenzhong Duan wrote:
> There are cases where a guest tries to switch spinlocks to bare metal
> behavior (e.g. by setting "xen_nopvspin" on XEN platform and
> "hv_nopvspin" on HYPER_V).
> 
> That feature is missed on KVM, add a new parameter "nopvspin" to disable
> PV spinlocks for KVM guest.
> 
> This new parameter is also used to replace "xen_nopvspin" and
> "hv_nopvspin".

This is confusing as there are no Xen or Hyper-V changes in this patch.
Please make it clear that you're talking about future patches, e.g.:

  The new 'nopvspin' parameter will also replace Xen and Hyper-V specific
  parameters in future patches.

> 
> The global variable pvspin isn't defined as __initdata as it's used at
> runtime by XEN guest.

Same comment as above regarding what this patch is doing versus what will
be done in the future.  Arguably you should even mark it __initdata in
this patch and deal with conflict in the Xen patch, e.g. use it only to
set the existing xen_pvspin variable.

> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krcmar <rkrcmar@redhat.com>
> Cc: Sean Christopherson <sean.j.christopherson@intel.com>
> Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
> Cc: Wanpeng Li <wanpengli@tencent.com>
> Cc: Jim Mattson <jmattson@google.com>
> Cc: Joerg Roedel <joro@8bytes.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Will Deacon <will@kernel.org>
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 4 ++++
>  arch/x86/include/asm/qspinlock.h                | 1 +
>  arch/x86/kernel/kvm.c                           | 7 +++++++
>  kernel/locking/qspinlock.c                      | 7 +++++++
>  4 files changed, 19 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index c7ac2f3..4b956d8 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5330,6 +5330,10 @@
>  			as generic guest with no PV drivers. Currently support
>  			XEN HVM, KVM, HYPER_V and VMWARE guest.
>  
> +	nopvspin	[X86,KVM] Disables the qspinlock slow path
> +			using PV optimizations which allow the hypervisor to
> +			'idle' the guest on lock contention.
> +
>  	xirc2ps_cs=	[NET,PCMCIA]
>  			Format:
>  			<irq>,<irq_mask>,<io>,<full_duplex>,<do_sound>,<lockup_hack>[,<irq2>[,<irq3>[,<irq4>]]]
> diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
> index 444d6fd..34a4484 100644
> --- a/arch/x86/include/asm/qspinlock.h
> +++ b/arch/x86/include/asm/qspinlock.h
> @@ -32,6 +32,7 @@ static __always_inline u32 queued_fetch_set_pending_acquire(struct qspinlock *lo
>  extern void __pv_init_lock_hash(void);
>  extern void __pv_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
>  extern void __raw_callee_save___pv_queued_spin_unlock(struct qspinlock *lock);
> +extern bool pvspin;
>  
>  #define	queued_spin_unlock queued_spin_unlock
>  /**
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index e820568..a4f108d 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -842,6 +842,13 @@ void __init kvm_spinlock_init(void)
>  	if (num_possible_cpus() == 1)
>  		return;
>  
> +	if (!pvspin) {
> +		pr_info("PV spinlocks disabled\n");
> +		static_branch_disable(&virt_spin_lock_key);
> +		return;
> +	}
> +	pr_info("PV spinlocks enabled\n");

These prints could be confusing as KVM also disables PV spinlocks when it
sees KVM_HINTS_REALTIME.

> +
>  	__pv_init_lock_hash();
>  	pv_ops.lock.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
>  	pv_ops.lock.queued_spin_unlock =
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 2473f10..945b510 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -580,4 +580,11 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>  #include "qspinlock_paravirt.h"
>  #include "qspinlock.c"
>  
> +bool pvspin = true;

This can be __ro_after_init, or probably better __initdata and have Xen
snapshot the value for its use case.

Personal preference: I'd invert the bool and name it nopvspin to make it
easier to connect the variable to the kernel param.

> +static __init int parse_nopvspin(char *arg)
> +{
> +	pvspin = false;
> +	return 0;
> +}
> +early_param("nopvspin", parse_nopvspin);
>  #endif
> -- 
> 1.8.3.1
>
Zhenzhong Duan Oct. 3, 2019, 10:30 a.m. UTC | #2
On 2019/10/3 1:10, Sean Christopherson wrote:

> On Mon, Sep 30, 2019 at 08:44:36PM +0800, Zhenzhong Duan wrote:
>> There are cases where a guest tries to switch spinlocks to bare metal
>> behavior (e.g. by setting "xen_nopvspin" on XEN platform and
>> "hv_nopvspin" on HYPER_V).
>>
>> That feature is missed on KVM, add a new parameter "nopvspin" to disable
>> PV spinlocks for KVM guest.
>>
>> This new parameter is also used to replace "xen_nopvspin" and
>> "hv_nopvspin".
> This is confusing as there are no Xen or Hyper-V changes in this patch.
> Please make it clear that you're talking about future patches, e.g.:
>
>    The new 'nopvspin' parameter will also replace Xen and Hyper-V specific
>    parameters in future patches.

Will fix

>
>> The global variable pvspin isn't defined as __initdata as it's used at
>> runtime by XEN guest.
> Same comment as above regarding what this patch is doing versus what will
> be done in the future.  Arguably you should even mark it __initdata in
> this patch and deal with conflict in the Xen patch, e.g. use it only to
> set the existing xen_pvspin variable.

Will fix

>
>> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>

......snip

>>   /**
>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> index e820568..a4f108d 100644
>> --- a/arch/x86/kernel/kvm.c
>> +++ b/arch/x86/kernel/kvm.c
>> @@ -842,6 +842,13 @@ void __init kvm_spinlock_init(void)
>>   	if (num_possible_cpus() == 1)
>>   		return;
>>   
>> +	if (!pvspin) {
>> +		pr_info("PV spinlocks disabled\n");
>> +		static_branch_disable(&virt_spin_lock_key);
>> +		return;
>> +	}
>> +	pr_info("PV spinlocks enabled\n");
> These prints could be confusing as KVM also disables PV spinlocks when it
> sees KVM_HINTS_REALTIME.

What about below:

pr_info("PV spinlocks disabled forced by \"nopvspin\" parameter.\n");

Or you prefer separate print for each disabling like below?

         /* Does host kernel support KVM_FEATURE_PV_UNHALT? */
         if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)) {
                 pr_info("PV spinlocks disabled, KVM_FEATURE_PV_UNHALT feature needed.\n");
                 return;
         }

         if (kvm_para_has_hint(KVM_HINTS_REALTIME)) {
                 pr_info("PV spinlocks disabled, having non-preemption hints.\n");
                 return;
         }

         /* Don't use the pvqspinlock code if there is only 1 vCPU. */
         if (num_possible_cpus() == 1) {
                 pr_info("PV spinlocks disabled on UP.\n");
                 return;
         }
	if (!pvspin) {
		pr_info("PV spinlocks disabled forced by \"nopvspin\" parameter.\n");
		static_branch_disable(&virt_spin_lock_key);
		return;
	}
	pr_info("PV spinlocks enabled\n");

>
>> +
>>   	__pv_init_lock_hash();
>>   	pv_ops.lock.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
>>   	pv_ops.lock.queued_spin_unlock =
>> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
>> index 2473f10..945b510 100644
>> --- a/kernel/locking/qspinlock.c
>> +++ b/kernel/locking/qspinlock.c
>> @@ -580,4 +580,11 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>>   #include "qspinlock_paravirt.h"
>>   #include "qspinlock.c"
>>   
>> +bool pvspin = true;
> This can be __ro_after_init, or probably better __initdata and have Xen
> snapshot the value for its use case.

I will use __initdata

>
> Personal preference: I'd invert the bool and name it nopvspin to make it
> easier to connect the variable to the kernel param.

OK, will do that.  Thanks for review for all the patches.

Zhenzhong

Patch
diff mbox series

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index c7ac2f3..4b956d8 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5330,6 +5330,10 @@ 
 			as generic guest with no PV drivers. Currently support
 			XEN HVM, KVM, HYPER_V and VMWARE guest.
 
+	nopvspin	[X86,KVM] Disables the qspinlock slow path
+			using PV optimizations which allow the hypervisor to
+			'idle' the guest on lock contention.
+
 	xirc2ps_cs=	[NET,PCMCIA]
 			Format:
 			<irq>,<irq_mask>,<io>,<full_duplex>,<do_sound>,<lockup_hack>[,<irq2>[,<irq3>[,<irq4>]]]
diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
index 444d6fd..34a4484 100644
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -32,6 +32,7 @@  static __always_inline u32 queued_fetch_set_pending_acquire(struct qspinlock *lo
 extern void __pv_init_lock_hash(void);
 extern void __pv_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
 extern void __raw_callee_save___pv_queued_spin_unlock(struct qspinlock *lock);
+extern bool pvspin;
 
 #define	queued_spin_unlock queued_spin_unlock
 /**
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index e820568..a4f108d 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -842,6 +842,13 @@  void __init kvm_spinlock_init(void)
 	if (num_possible_cpus() == 1)
 		return;
 
+	if (!pvspin) {
+		pr_info("PV spinlocks disabled\n");
+		static_branch_disable(&virt_spin_lock_key);
+		return;
+	}
+	pr_info("PV spinlocks enabled\n");
+
 	__pv_init_lock_hash();
 	pv_ops.lock.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
 	pv_ops.lock.queued_spin_unlock =
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 2473f10..945b510 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -580,4 +580,11 @@  void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 #include "qspinlock_paravirt.h"
 #include "qspinlock.c"
 
+bool pvspin = true;
+static __init int parse_nopvspin(char *arg)
+{
+	pvspin = false;
+	return 0;
+}
+early_param("nopvspin", parse_nopvspin);
 #endif