Re: [PATCH 1/2] KVM: arm: rename pause into power_off

[Eric Auger <eric.auger@linaro.org>]

Hi Christoffer,
On 07/18/2015 11:09 AM, Christoffer Dall wrote:
> On Mon, Jul 06, 2015 at 02:49:55PM +0200, Eric Auger wrote:
>> The kvm_vcpu_arch pause field is renamed into power_off to prepare
>> for the introduction of a new pause field.
>>
>> Signed-off-by: Eric Auger <eric.auger@linaro.org>
>>
>> v4 -> v5:
>> - fix compilation issue on arm64 (add power_off field in kvm_host.h)
>> ---
>>  arch/arm/include/asm/kvm_host.h   |  4 ++--
>>  arch/arm/kvm/arm.c                | 10 +++++-----
>>  arch/arm/kvm/psci.c               | 10 +++++-----
>>  arch/arm64/include/asm/kvm_host.h |  4 ++--
>>  4 files changed, 14 insertions(+), 14 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
>> index e896d2c..304004d 100644
>> --- a/arch/arm/include/asm/kvm_host.h
>> +++ b/arch/arm/include/asm/kvm_host.h
>> @@ -129,8 +129,8 @@ struct kvm_vcpu_arch {
>>  	 * here.
>>  	 */
>>  
>> -	/* Don't run the guest on this vcpu */
>> -	bool pause;
>> +	/* vcpu power-off state */
>> +	bool power_off;
>>  
>>  	/* IO related fields */
>>  	struct kvm_decode mmio_decode;
>> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
>> index bcdf799..7537e68 100644
>> --- a/arch/arm/kvm/arm.c
>> +++ b/arch/arm/kvm/arm.c
>> @@ -475,7 +475,7 @@ static void vcpu_pause(struct kvm_vcpu *vcpu)
>>  {
>>  	wait_queue_head_t *wq = kvm_arch_vcpu_wq(vcpu);
>>  
>> -	wait_event_interruptible(*wq, !vcpu->arch.pause);
>> +	wait_event_interruptible(*wq, !vcpu->arch.power_off);
> 
> would there be any benefit to simply calling kvm_vcpu_block() instead of
> vcpu_pause, and rewrite kvm_arch_vcpu_runnable to:
Wouldn't it somehow change the known behavior or kvm_vcpu_block which is
expected/used to exit on IRQ/FIQ (WFI). Here it would exit when
power_off changes to false (or maybe you meant pause below in the new
context?).
> 
> int kvm_arch_vcpu_runnable(struct kvm_vcpu *v)
> {
> ▸       return !vcpu->arch.power_off &&
> 		(!!v->arch.irq_lines || kvm_vgic_vcpu_pending_irq(v));
> }
> 
> Not sure really, certainly the runnable function does not become more
> readable.
To me the usage of kvm_vcpu_block looks more complex than this code and
I would prefer keeping that version if you don't mind.
> 
>>  }
>>  
>>  static int kvm_vcpu_initialized(struct kvm_vcpu *vcpu)
>> @@ -525,7 +525,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
>>  
>>  		update_vttbr(vcpu->kvm);
>>  
>> -		if (vcpu->arch.pause)
>> +		if (vcpu->arch.power_off)
>>  			vcpu_pause(vcpu);
> 
> looking back over this code, how does this actually guarantee that we
> don't run a powered-off cpu?
> 
> vcpu_pause() just does a wait_event_interruptible(), so if we get
> scheduled again, we'll just proceed running. 
actually it also checks the !vcpu->arch.power_off condition, right?
 Is there any case where we
> could get scheduled without signal_pending() being true and therefore
> inadvertedly run the vcpu?
kvm_arm_halt_guest can happen at any time, including after the execution
of above vcpu_pause(vcpu) call. This is the reason why I added the
second check below, once we entered the critical section and just before
running the vcpu.

With regard to renamed power_off boolean my understanding is:

power_off is set
- on kvm_arch_vcpu_ioctl_vcpu_init/KVM_ARM_VCPU_POWER_OFF
- on PSCI calls from guests through traps from HVC instructions
in that case I don't think this can happen

- on KVM_SET_MP_STATE ioctl: I think in that case, what you describe can
happen.

Do you share the same understanding?

Best Regards

Eric

> 
> if so, we should change the line below like this:
> 
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index bc738d2..98f31e6 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -542,7 +542,8 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
>  			run->exit_reason = KVM_EXIT_INTR;
>  		}
>  
> -		if (ret <= 0 || need_new_vmid_gen(vcpu->kvm)) {
> +		if (ret <= 0 || need_new_vmid_gen(vcpu->kvm) ||
> +		    vcpu->arch.power_off) {
>  			local_irq_enable();
>  			preempt_enable();
>  			kvm_timer_sync_hwstate(vcpu);
> 
> 
> Sorry for polluting your patch with these questions, I'm otherwise fine
> with the rename.
> 
> Thanks,
> -Christoffer
> 
>>  
>>  		/*
>> @@ -766,12 +766,12 @@ static int kvm_arch_vcpu_ioctl_vcpu_init(struct kvm_vcpu *vcpu,
>>  	vcpu_reset_hcr(vcpu);
>>  
>>  	/*
>> -	 * Handle the "start in power-off" case by marking the VCPU as paused.
>> +	 * Handle the "start in power-off" case.
>>  	 */
>>  	if (test_bit(KVM_ARM_VCPU_POWER_OFF, vcpu->arch.features))
>> -		vcpu->arch.pause = true;
>> +		vcpu->arch.power_off = true;
>>  	else
>> -		vcpu->arch.pause = false;
>> +		vcpu->arch.power_off = false;
>>  
>>  	return 0;
>>  }
>> diff --git a/arch/arm/kvm/psci.c b/arch/arm/kvm/psci.c
>> index 4b94b51..134971a 100644
>> --- a/arch/arm/kvm/psci.c
>> +++ b/arch/arm/kvm/psci.c
>> @@ -63,7 +63,7 @@ static unsigned long kvm_psci_vcpu_suspend(struct kvm_vcpu *vcpu)
>>  
>>  static void kvm_psci_vcpu_off(struct kvm_vcpu *vcpu)
>>  {
>> -	vcpu->arch.pause = true;
>> +	vcpu->arch.power_off = true;
>>  }
>>  
>>  static unsigned long kvm_psci_vcpu_on(struct kvm_vcpu *source_vcpu)
>> @@ -87,7 +87,7 @@ static unsigned long kvm_psci_vcpu_on(struct kvm_vcpu *source_vcpu)
>>  	 */
>>  	if (!vcpu)
>>  		return PSCI_RET_INVALID_PARAMS;
>> -	if (!vcpu->arch.pause) {
>> +	if (!vcpu->arch.power_off) {
>>  		if (kvm_psci_version(source_vcpu) != KVM_ARM_PSCI_0_1)
>>  			return PSCI_RET_ALREADY_ON;
>>  		else
>> @@ -115,7 +115,7 @@ static unsigned long kvm_psci_vcpu_on(struct kvm_vcpu *source_vcpu)
>>  	 * the general puspose registers are undefined upon CPU_ON.
>>  	 */
>>  	*vcpu_reg(vcpu, 0) = context_id;
>> -	vcpu->arch.pause = false;
>> +	vcpu->arch.power_off = false;
>>  	smp_mb();		/* Make sure the above is visible */
>>  
>>  	wq = kvm_arch_vcpu_wq(vcpu);
>> @@ -152,7 +152,7 @@ static unsigned long kvm_psci_vcpu_affinity_info(struct kvm_vcpu *vcpu)
>>  	kvm_for_each_vcpu(i, tmp, kvm) {
>>  		mpidr = kvm_vcpu_get_mpidr_aff(tmp);
>>  		if (((mpidr & target_affinity_mask) == target_affinity) &&
>> -		    !tmp->arch.pause) {
>> +		    !tmp->arch.power_off) {
>>  			return PSCI_0_2_AFFINITY_LEVEL_ON;
>>  		}
>>  	}
>> @@ -175,7 +175,7 @@ static void kvm_prepare_system_event(struct kvm_vcpu *vcpu, u32 type)
>>  	 * re-initialized.
>>  	 */
>>  	kvm_for_each_vcpu(i, tmp, vcpu->kvm) {
>> -		tmp->arch.pause = true;
>> +		tmp->arch.power_off = true;
>>  		kvm_vcpu_kick(tmp);
>>  	}
>>  
>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>> index 2709db2..009da6b 100644
>> --- a/arch/arm64/include/asm/kvm_host.h
>> +++ b/arch/arm64/include/asm/kvm_host.h
>> @@ -122,8 +122,8 @@ struct kvm_vcpu_arch {
>>  	 * here.
>>  	 */
>>  
>> -	/* Don't run the guest */
>> -	bool pause;
>> +	/* vcpu power-off state */
>> +	bool power_off;
>>  
>>  	/* IO related fields */
>>  	struct kvm_decode mmio_decode;
>> -- 
>> 1.9.1
>>

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