[v7,3/5] x86/kvm: Add "nopvspin" parameter to disable PV spinlocks
diff mbox series

Message ID 1571649076-2421-4-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 Oct. 21, 2019, 9:11 a.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.

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

Define variable nopvsin as global because it will be used in future
patches as above.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.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 |  5 ++++
 arch/x86/include/asm/qspinlock.h                |  1 +
 arch/x86/kernel/kvm.c                           | 34 ++++++++++++++++++++-----
 kernel/locking/qspinlock.c                      |  7 +++++
 4 files changed, 40 insertions(+), 7 deletions(-)

Comments

Vitaly Kuznetsov Oct. 22, 2019, 11:36 a.m. UTC | #1
Zhenzhong Duan <zhenzhong.duan@oracle.com> writes:

> 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.
>
> The new 'nopvspin' parameter will also replace Xen and Hyper-V specific
> parameters in future patches.
>
> Define variable nopvsin as global because it will be used in future
> patches as above.
>
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
> Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.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 |  5 ++++
>  arch/x86/include/asm/qspinlock.h                |  1 +
>  arch/x86/kernel/kvm.c                           | 34 ++++++++++++++++++++-----
>  kernel/locking/qspinlock.c                      |  7 +++++
>  4 files changed, 40 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index a84a83f..bd49ed2 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5334,6 +5334,11 @@
>  			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..d86ab94 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 nopvspin;
>  
>  #define	queued_spin_unlock queued_spin_unlock
>  /**
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 249f14a..3945aa5 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -825,18 +825,36 @@ __visible bool __kvm_vcpu_is_preempted(long cpu)
>   */
>  void __init kvm_spinlock_init(void)
>  {
> -	/* Does host kernel support KVM_FEATURE_PV_UNHALT? */
> -	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
> +	/*
> +	 * In case host doesn't support KVM_FEATURE_PV_UNHALT there is still an
> +	 * advantage of keeping virt_spin_lock_key enabled: virt_spin_lock() is
> +	 * preferred over native qspinlock when vCPU is preempted.
> +	 */
> +	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)) {
> +		pr_info("PV spinlocks disabled, no host support.\n");
>  		return;
> +	}
>  
> +	/*
> +	 * Disable PV qspinlock and use native qspinlock when dedicated pCPUs
> +	 * are available.
> +	 */
>  	if (kvm_para_has_hint(KVM_HINTS_REALTIME)) {
> -		static_branch_disable(&virt_spin_lock_key);
> -		return;
> +		pr_info("PV spinlocks disabled with KVM_HINTS_REALTIME hints.\n");
> +		goto out;
>  	}
>  
> -	/* Don't use the pvqspinlock code if there is only 1 vCPU. */
> -	if (num_possible_cpus() == 1)
> -		return;
> +	if (num_possible_cpus() == 1) {
> +		pr_info("PV spinlocks disabled, single CPU.\n");
> +		goto out;
> +	}
> +
> +	if (nopvspin) {
> +		pr_info("PV spinlocks disabled, forced by \"nopvspin\" parameter.\n");
> +		goto out;
> +	}
> +
> +	pr_info("PV spinlocks enabled\n");
>  
>  	__pv_init_lock_hash();
>  	pv_ops.lock.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
> @@ -849,6 +867,8 @@ void __init kvm_spinlock_init(void)
>  		pv_ops.lock.vcpu_is_preempted =
>  			PV_CALLEE_SAVE(__kvm_vcpu_is_preempted);
>  	}
> +out:
> +	static_branch_disable(&virt_spin_lock_key);

You probably need to add 'return' before 'out:' as it seems you're
disabling virt_spin_lock_key in all cases now).

>  }
>  
>  #endif	/* CONFIG_PARAVIRT_SPINLOCKS */
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 2473f10..75193d6 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 nopvspin __initdata;
> +static __init int parse_nopvspin(char *arg)
> +{
> +	nopvspin = true;
> +	return 0;
> +}
> +early_param("nopvspin", parse_nopvspin);
>  #endif
Zhenzhong Duan Oct. 22, 2019, 12:46 p.m. UTC | #2
Hi Vitaly,

On 2019/10/22 19:36, Vitaly Kuznetsov wrote:

> Zhenzhong Duan<zhenzhong.duan@oracle.com>  writes:
>
...snip

>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> index 249f14a..3945aa5 100644
>> --- a/arch/x86/kernel/kvm.c
>> +++ b/arch/x86/kernel/kvm.c
>> @@ -825,18 +825,36 @@ __visible bool __kvm_vcpu_is_preempted(long cpu)
>>    */
>>   void __init kvm_spinlock_init(void)
>>   {
>> -	/* Does host kernel support KVM_FEATURE_PV_UNHALT? */
>> -	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
>> +	/*
>> +	 * In case host doesn't support KVM_FEATURE_PV_UNHALT there is still an
>> +	 * advantage of keeping virt_spin_lock_key enabled: virt_spin_lock() is
>> +	 * preferred over native qspinlock when vCPU is preempted.
>> +	 */
>> +	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)) {
>> +		pr_info("PV spinlocks disabled, no host support.\n");
>>   		return;
>> +	}
>>   
>> +	/*
>> +	 * Disable PV qspinlock and use native qspinlock when dedicated pCPUs
>> +	 * are available.
>> +	 */
>>   	if (kvm_para_has_hint(KVM_HINTS_REALTIME)) {
>> -		static_branch_disable(&virt_spin_lock_key);
>> -		return;
>> +		pr_info("PV spinlocks disabled with KVM_HINTS_REALTIME hints.\n");
>> +		goto out;
>>   	}
>>   
>> -	/* Don't use the pvqspinlock code if there is only 1 vCPU. */
>> -	if (num_possible_cpus() == 1)
>> -		return;
>> +	if (num_possible_cpus() == 1) {
>> +		pr_info("PV spinlocks disabled, single CPU.\n");
>> +		goto out;
>> +	}
>> +
>> +	if (nopvspin) {
>> +		pr_info("PV spinlocks disabled, forced by \"nopvspin\" parameter.\n");
>> +		goto out;
>> +	}
>> +
>> +	pr_info("PV spinlocks enabled\n");
>>   
>>   	__pv_init_lock_hash();
>>   	pv_ops.lock.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
>> @@ -849,6 +867,8 @@ void __init kvm_spinlock_init(void)
>>   		pv_ops.lock.vcpu_is_preempted =
>>   			PV_CALLEE_SAVE(__kvm_vcpu_is_preempted);
>>   	}
>> +out:
>> +	static_branch_disable(&virt_spin_lock_key);
> You probably need to add 'return' before 'out:' as it seems you're
> disabling virt_spin_lock_key in all cases now).

virt_spin_lock_key is kept enabled in !kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)
case which is the only case virt_spin_lock() optimization is used.

When PV qspinlock is enabled, virt_spin_lock() isn't called in
__pv_queued_spin_lock_slowpath() in which case we don't care
virt_spin_lock_key's value.

So adding 'return' or not are both ok, I chosed to save a line,
let me know if you prefer to add a 'return' and I'll change it.

btw: __pv_queued_spin_lock_slowpath() is alias of queued_spin_lock_slowpath()

Thanks
Zhenzhong
Vitaly Kuznetsov Oct. 22, 2019, 1:11 p.m. UTC | #3
Zhenzhong Duan <zhenzhong.duan@oracle.com> writes:

> Hi Vitaly,
>
> On 2019/10/22 19:36, Vitaly Kuznetsov wrote:
>
>> Zhenzhong Duan<zhenzhong.duan@oracle.com>  writes:
>>
> ...snip
>
>>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>>> index 249f14a..3945aa5 100644
>>> --- a/arch/x86/kernel/kvm.c
>>> +++ b/arch/x86/kernel/kvm.c
>>> @@ -825,18 +825,36 @@ __visible bool __kvm_vcpu_is_preempted(long cpu)
>>>    */
>>>   void __init kvm_spinlock_init(void)
>>>   {
>>> -	/* Does host kernel support KVM_FEATURE_PV_UNHALT? */
>>> -	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
>>> +	/*
>>> +	 * In case host doesn't support KVM_FEATURE_PV_UNHALT there is still an
>>> +	 * advantage of keeping virt_spin_lock_key enabled: virt_spin_lock() is
>>> +	 * preferred over native qspinlock when vCPU is preempted.
>>> +	 */
>>> +	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)) {
>>> +		pr_info("PV spinlocks disabled, no host support.\n");
>>>   		return;
>>> +	}
>>>   
>>> +	/*
>>> +	 * Disable PV qspinlock and use native qspinlock when dedicated pCPUs
>>> +	 * are available.
>>> +	 */
>>>   	if (kvm_para_has_hint(KVM_HINTS_REALTIME)) {
>>> -		static_branch_disable(&virt_spin_lock_key);
>>> -		return;
>>> +		pr_info("PV spinlocks disabled with KVM_HINTS_REALTIME hints.\n");
>>> +		goto out;
>>>   	}
>>>   
>>> -	/* Don't use the pvqspinlock code if there is only 1 vCPU. */
>>> -	if (num_possible_cpus() == 1)
>>> -		return;
>>> +	if (num_possible_cpus() == 1) {
>>> +		pr_info("PV spinlocks disabled, single CPU.\n");
>>> +		goto out;
>>> +	}
>>> +
>>> +	if (nopvspin) {
>>> +		pr_info("PV spinlocks disabled, forced by \"nopvspin\" parameter.\n");
>>> +		goto out;
>>> +	}
>>> +
>>> +	pr_info("PV spinlocks enabled\n");
>>>   
>>>   	__pv_init_lock_hash();
>>>   	pv_ops.lock.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
>>> @@ -849,6 +867,8 @@ void __init kvm_spinlock_init(void)
>>>   		pv_ops.lock.vcpu_is_preempted =
>>>   			PV_CALLEE_SAVE(__kvm_vcpu_is_preempted);
>>>   	}
>>> +out:
>>> +	static_branch_disable(&virt_spin_lock_key);
>> You probably need to add 'return' before 'out:' as it seems you're
>> disabling virt_spin_lock_key in all cases now).
>
> virt_spin_lock_key is kept enabled in !kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)
> case which is the only case virt_spin_lock() optimization is used.
>
> When PV qspinlock is enabled, virt_spin_lock() isn't called in
> __pv_queued_spin_lock_slowpath() in which case we don't care
> virt_spin_lock_key's value.
>

True, my bad: I though we still need it enabled for something.

> So adding 'return' or not are both ok, I chosed to save a line,
> let me know if you prefer to add a 'return' and I'll change it.

No, please ignore.

>
> btw: __pv_queued_spin_lock_slowpath() is alias of queued_spin_lock_slowpath()
>
> Thanks
> Zhenzhong
>
Sean Christopherson Oct. 22, 2019, 9:03 p.m. UTC | #4
On Tue, Oct 22, 2019 at 08:46:46PM +0800, Zhenzhong Duan wrote:
> Hi Vitaly,
> 
> On 2019/10/22 19:36, Vitaly Kuznetsov wrote:
> 
> >Zhenzhong Duan<zhenzhong.duan@oracle.com>  writes:
> >
> ...snip
> 
> >>diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> >>index 249f14a..3945aa5 100644
> >>--- a/arch/x86/kernel/kvm.c
> >>+++ b/arch/x86/kernel/kvm.c
> >>@@ -825,18 +825,36 @@ __visible bool __kvm_vcpu_is_preempted(long cpu)
> >>   */
> >>  void __init kvm_spinlock_init(void)
> >>  {
> >>-	/* Does host kernel support KVM_FEATURE_PV_UNHALT? */
> >>-	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
> >>+	/*
> >>+	 * In case host doesn't support KVM_FEATURE_PV_UNHALT there is still an
> >>+	 * advantage of keeping virt_spin_lock_key enabled: virt_spin_lock() is
> >>+	 * preferred over native qspinlock when vCPU is preempted.
> >>+	 */
> >>+	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)) {
> >>+		pr_info("PV spinlocks disabled, no host support.\n");
> >>  		return;
> >>+	}
> >>+	/*
> >>+	 * Disable PV qspinlock and use native qspinlock when dedicated pCPUs
> >>+	 * are available.
> >>+	 */
> >>  	if (kvm_para_has_hint(KVM_HINTS_REALTIME)) {
> >>-		static_branch_disable(&virt_spin_lock_key);
> >>-		return;
> >>+		pr_info("PV spinlocks disabled with KVM_HINTS_REALTIME hints.\n");
> >>+		goto out;
> >>  	}
> >>-	/* Don't use the pvqspinlock code if there is only 1 vCPU. */
> >>-	if (num_possible_cpus() == 1)
> >>-		return;
> >>+	if (num_possible_cpus() == 1) {
> >>+		pr_info("PV spinlocks disabled, single CPU.\n");
> >>+		goto out;
> >>+	}
> >>+
> >>+	if (nopvspin) {
> >>+		pr_info("PV spinlocks disabled, forced by \"nopvspin\" parameter.\n");
> >>+		goto out;
> >>+	}
> >>+
> >>+	pr_info("PV spinlocks enabled\n");
> >>  	__pv_init_lock_hash();
> >>  	pv_ops.lock.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
> >>@@ -849,6 +867,8 @@ void __init kvm_spinlock_init(void)
> >>  		pv_ops.lock.vcpu_is_preempted =
> >>  			PV_CALLEE_SAVE(__kvm_vcpu_is_preempted);
> >>  	}
> >>+out:
> >>+	static_branch_disable(&virt_spin_lock_key);
> >You probably need to add 'return' before 'out:' as it seems you're
> >disabling virt_spin_lock_key in all cases now).
> 
> virt_spin_lock_key is kept enabled in !kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)
> case which is the only case virt_spin_lock() optimization is used.
> 
> When PV qspinlock is enabled, virt_spin_lock() isn't called in
> __pv_queued_spin_lock_slowpath() in which case we don't care
> virt_spin_lock_key's value.
> 
> So adding 'return' or not are both ok, I chosed to save a line,
> let me know if you prefer to add a 'return' and I'll change it.

It'd be worth adding a comment here if you end up spinning another version
to change the logging prefix.  The logic is sound and I like the end
result, but I had the same knee jerk "this can't be right!?!?" reaction as
Vitaly.
Zhenzhong Duan Oct. 23, 2019, 1:36 a.m. UTC | #5
On 2019/10/23 5:03, Sean Christopherson wrote:
> On Tue, Oct 22, 2019 at 08:46:46PM +0800, Zhenzhong Duan wrote:
>> Hi Vitaly,
>>
>> On 2019/10/22 19:36, Vitaly Kuznetsov wrote:
>>
>>> Zhenzhong Duan<zhenzhong.duan@oracle.com>  writes:
>>>
>> ...snip
>>
>>>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>>>> index 249f14a..3945aa5 100644
>>>> --- a/arch/x86/kernel/kvm.c
>>>> +++ b/arch/x86/kernel/kvm.c
>>>> @@ -825,18 +825,36 @@ __visible bool __kvm_vcpu_is_preempted(long cpu)
>>>>    */
>>>>   void __init kvm_spinlock_init(void)
>>>>   {
>>>> -	/* Does host kernel support KVM_FEATURE_PV_UNHALT? */
>>>> -	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
>>>> +	/*
>>>> +	 * In case host doesn't support KVM_FEATURE_PV_UNHALT there is still an
>>>> +	 * advantage of keeping virt_spin_lock_key enabled: virt_spin_lock() is
>>>> +	 * preferred over native qspinlock when vCPU is preempted.
>>>> +	 */
>>>> +	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)) {
>>>> +		pr_info("PV spinlocks disabled, no host support.\n");
>>>>   		return;
>>>> +	}
>>>> +	/*
>>>> +	 * Disable PV qspinlock and use native qspinlock when dedicated pCPUs
>>>> +	 * are available.
>>>> +	 */
>>>>   	if (kvm_para_has_hint(KVM_HINTS_REALTIME)) {
>>>> -		static_branch_disable(&virt_spin_lock_key);
>>>> -		return;
>>>> +		pr_info("PV spinlocks disabled with KVM_HINTS_REALTIME hints.\n");
>>>> +		goto out;
>>>>   	}
>>>> -	/* Don't use the pvqspinlock code if there is only 1 vCPU. */
>>>> -	if (num_possible_cpus() == 1)
>>>> -		return;
>>>> +	if (num_possible_cpus() == 1) {
>>>> +		pr_info("PV spinlocks disabled, single CPU.\n");
>>>> +		goto out;
>>>> +	}
>>>> +
>>>> +	if (nopvspin) {
>>>> +		pr_info("PV spinlocks disabled, forced by \"nopvspin\" parameter.\n");
>>>> +		goto out;
>>>> +	}
>>>> +
>>>> +	pr_info("PV spinlocks enabled\n");
>>>>   	__pv_init_lock_hash();
>>>>   	pv_ops.lock.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
>>>> @@ -849,6 +867,8 @@ void __init kvm_spinlock_init(void)
>>>>   		pv_ops.lock.vcpu_is_preempted =
>>>>   			PV_CALLEE_SAVE(__kvm_vcpu_is_preempted);
>>>>   	}
>>>> +out:
>>>> +	static_branch_disable(&virt_spin_lock_key);
>>> You probably need to add 'return' before 'out:' as it seems you're
>>> disabling virt_spin_lock_key in all cases now).
>> virt_spin_lock_key is kept enabled in !kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)
>> case which is the only case virt_spin_lock() optimization is used.
>>
>> When PV qspinlock is enabled, virt_spin_lock() isn't called in
>> __pv_queued_spin_lock_slowpath() in which case we don't care
>> virt_spin_lock_key's value.
>>
>> So adding 'return' or not are both ok, I chosed to save a line,
>> let me know if you prefer to add a 'return' and I'll change it.
> It'd be worth adding a comment here if you end up spinning another version
> to change the logging prefix.  The logic is sound and I like the end
> result, but I had the same knee jerk "this can't be right!?!?" reaction as
> Vitaly.

Sure, will do in next version.

Thanks

Zhenzhong

Patch
diff mbox series

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a84a83f..bd49ed2 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5334,6 +5334,11 @@ 
 			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..d86ab94 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 nopvspin;
 
 #define	queued_spin_unlock queued_spin_unlock
 /**
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 249f14a..3945aa5 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -825,18 +825,36 @@  __visible bool __kvm_vcpu_is_preempted(long cpu)
  */
 void __init kvm_spinlock_init(void)
 {
-	/* Does host kernel support KVM_FEATURE_PV_UNHALT? */
-	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
+	/*
+	 * In case host doesn't support KVM_FEATURE_PV_UNHALT there is still an
+	 * advantage of keeping virt_spin_lock_key enabled: virt_spin_lock() is
+	 * preferred over native qspinlock when vCPU is preempted.
+	 */
+	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)) {
+		pr_info("PV spinlocks disabled, no host support.\n");
 		return;
+	}
 
+	/*
+	 * Disable PV qspinlock and use native qspinlock when dedicated pCPUs
+	 * are available.
+	 */
 	if (kvm_para_has_hint(KVM_HINTS_REALTIME)) {
-		static_branch_disable(&virt_spin_lock_key);
-		return;
+		pr_info("PV spinlocks disabled with KVM_HINTS_REALTIME hints.\n");
+		goto out;
 	}
 
-	/* Don't use the pvqspinlock code if there is only 1 vCPU. */
-	if (num_possible_cpus() == 1)
-		return;
+	if (num_possible_cpus() == 1) {
+		pr_info("PV spinlocks disabled, single CPU.\n");
+		goto out;
+	}
+
+	if (nopvspin) {
+		pr_info("PV spinlocks disabled, forced by \"nopvspin\" parameter.\n");
+		goto out;
+	}
+
+	pr_info("PV spinlocks enabled\n");
 
 	__pv_init_lock_hash();
 	pv_ops.lock.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
@@ -849,6 +867,8 @@  void __init kvm_spinlock_init(void)
 		pv_ops.lock.vcpu_is_preempted =
 			PV_CALLEE_SAVE(__kvm_vcpu_is_preempted);
 	}
+out:
+	static_branch_disable(&virt_spin_lock_key);
 }
 
 #endif	/* CONFIG_PARAVIRT_SPINLOCKS */
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 2473f10..75193d6 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 nopvspin __initdata;
+static __init int parse_nopvspin(char *arg)
+{
+	nopvspin = true;
+	return 0;
+}
+early_param("nopvspin", parse_nopvspin);
 #endif