Re: [PATCH 1/3] KVM: add spinlock-exiting optimize framework

[David Hildenbrand]

On 07.08.2017 10:44, Longpeng(Mike) wrote:
> If the vcpu(me) exit due to request a usermode spinlock, then
> the spinlock-holder may be preempted in usermode or kernmode.
> 
> But if the vcpu(me) is in kernmode, then the holder must be
> preempted in kernmode, so we should choose a vcpu in kernmode
> as the most eligible candidate.
> 
> For some architecture(e.g. arm/s390), spin/preempt_in_kernel()
> are the same, but they are different for X86.
> 
> Signed-off-by: Longpeng(Mike) <longpeng2@xxxxxxxxxx>
> ---
>  arch/mips/kvm/mips.c       | 10 ++++++++++
>  arch/powerpc/kvm/powerpc.c | 10 ++++++++++
>  arch/s390/kvm/kvm-s390.c   | 10 ++++++++++
>  arch/x86/kvm/x86.c         | 10 ++++++++++
>  include/linux/kvm_host.h   |  2 ++
>  virt/kvm/arm/arm.c         | 10 ++++++++++
>  virt/kvm/kvm_main.c        |  4 ++++
>  7 files changed, 56 insertions(+)
> 
> diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
> index d4b2ad1..e04e6b3 100644
> --- a/arch/mips/kvm/mips.c
> +++ b/arch/mips/kvm/mips.c
> @@ -98,6 +98,16 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
>  	return !!(vcpu->arch.pending_exceptions);
>  }
>  
> +bool kvm_arch_vcpu_spin_in_kernel(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
> +
> +bool kvm_arch_vcpu_preempt_in_kernel(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
> +
>  int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu)
>  {
>  	return 1;
> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> index 1a75c0b..c573ddd 100644
> --- a/arch/powerpc/kvm/powerpc.c
> +++ b/arch/powerpc/kvm/powerpc.c
> @@ -58,6 +58,16 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *v)
>  	return !!(v->arch.pending_exceptions) || kvm_request_pending(v);
>  }
>  
> +bool kvm_arch_vcpu_spin_in_kernel(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
> +
> +bool kvm_arch_vcpu_preempt_in_kernel(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
> +
>  int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu)
>  {
>  	return 1;
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index af09d34..f78cdc2 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -2447,6 +2447,16 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
>  	return kvm_s390_vcpu_has_irq(vcpu, 0);
>  }
>  
> +bool kvm_arch_vcpu_spin_in_kernel(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
> +
> +bool kvm_arch_vcpu_preempt_in_kernel(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
> +
>  void kvm_s390_vcpu_block(struct kvm_vcpu *vcpu)
>  {
>  	atomic_or(PROG_BLOCK_SIE, &vcpu->arch.sie_block->prog20);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 6c97c82..04c6a1f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8435,6 +8435,16 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
>  	return kvm_vcpu_running(vcpu) || kvm_vcpu_has_events(vcpu);
>  }
>  
> +bool kvm_arch_vcpu_spin_in_kernel(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
> +
> +bool kvm_arch_vcpu_preempt_in_kernel(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
> +
>  int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu)
>  {
>  	return kvm_vcpu_exiting_guest_mode(vcpu) == IN_GUEST_MODE;
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 890b706..9613620 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -798,6 +798,8 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
>  void kvm_arch_hardware_unsetup(void);
>  void kvm_arch_check_processor_compat(void *rtn);
>  int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu);
> +bool kvm_arch_vcpu_spin_in_kernel(struct kvm_vcpu *vcpu);
> +bool kvm_arch_vcpu_preempt_in_kernel(struct kvm_vcpu *vcpu);
>  int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu);
>  
>  #ifndef __KVM_HAVE_ARCH_VM_ALLOC
> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> index a39a1e1..e45f780 100644
> --- a/virt/kvm/arm/arm.c
> +++ b/virt/kvm/arm/arm.c
> @@ -416,6 +416,16 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *v)
>  		&& !v->arch.power_off && !v->arch.pause);
>  }
>  
> +bool kvm_arch_vcpu_spin_in_kernel(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
> +
> +bool kvm_arch_vcpu_preempt_in_kernel(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}

Is the differentiation really necessary?

Can't you cache for x86 in all scenarios and simply introduce
kvm_arch_vcpu_in_kernel() ?

Otherwise, we have complexity that might just be avoided (e.g.
kvm_arch_vcpu_spin_in_kernel must only be called on the loaded VCPU)

> +
>  /* Just ensure a guest exit from a particular CPU */
>  static void exit_vm_noop(void *info)
>  {
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index f3f7427..0d0527b 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2324,12 +2324,14 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
>  {
>  	struct kvm *kvm = me->kvm;
>  	struct kvm_vcpu *vcpu;
> +	bool in_kern;
>  	int last_boosted_vcpu = me->kvm->last_boosted_vcpu;
>  	int yielded = 0;
>  	int try = 3;
>  	int pass;
>  	int i;
>  
> +	in_kern = kvm_arch_vcpu_spin_in_kernel(me);
>  	kvm_vcpu_set_in_spin_loop(me, true);
>  	/*
>  	 * We boost the priority of a VCPU that is runnable but not
> @@ -2351,6 +2353,8 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
>  				continue;
>  			if (swait_active(&vcpu->wq) && !kvm_arch_vcpu_runnable(vcpu))
>  				continue;
> +			if (in_kern && !kvm_arch_vcpu_preempt_in_kernel(vcpu))
> +				continue;
>  			if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
>  				continue;
>  
> 


-- 

Thanks,

David