Subject: [PATCH 2/2] arm64: KVM: Yield CPU when vcpu executes a WFE

From: [email-masked] (Marc Zyngier)
Date: Mon, 07 Oct 2013 17:00:09 +0100

On 07/10/13 16:52, Bhushan Bharat-R65777 wrote:
> 
> 
>> -----Original Message-----
>> From: Marc Zyngier [mailto:marc.zyngier at arm.com]
>> Sent: Monday, October 07, 2013 9:11 PM
>> To: linux-arm-kernel at lists.infradead.org; kvmarm at lists.cs.columbia.edu;
>> kvm at vger.kernel.org
>> Subject: [PATCH 2/2] arm64: KVM: Yield CPU when vcpu executes a WFE
>>
>> On an (even slightly) oversubscribed system, spinlocks are quickly becoming a
>> bottleneck, as some vcpus are spinning, waiting for a lock to be released, while
>> the vcpu holding the lock may not be running at all.
>>
>> The solution is to trap blocking WFEs and tell KVM that we're now spinning. This
>> ensures that other vpus will get a scheduling boost, allowing the lock to be
>> released more quickly.
>>
>> Signed-off-by: Marc Zyngier <[email-masked]>
>> ---
>>  arch/arm64/include/asm/kvm_arm.h |  8 ++++++--
>>  arch/arm64/kvm/handle_exit.c     | 18 +++++++++++++-----
>>  2 files changed, 19 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
>> index a5f28e2..c98ef47 100644
>> --- a/arch/arm64/include/asm/kvm_arm.h
>> +++ b/arch/arm64/include/asm/kvm_arm.h
>> @@ -63,6 +63,7 @@
>>   * TAC:		Trap ACTLR
>>   * TSC:		Trap SMC
>>   * TSW:		Trap cache operations by set/way
>> + * TWE:		Trap WFE
>>   * TWI:		Trap WFI
>>   * TIDCP:	Trap L2CTLR/L2ECTLR
>>   * BSU_IS:	Upgrade barriers to the inner shareable domain
>> @@ -72,8 +73,9 @@
>>   * FMO:		Override CPSR.F and enable signaling with VF
>>   * SWIO:	Turn set/way invalidates into set/way clean+invalidate
>>   */
>> -#define HCR_GUEST_FLAGS (HCR_TSC | HCR_TSW | HCR_TWI | HCR_VM | HCR_BSU_IS | \
>> -			 HCR_FB | HCR_TAC | HCR_AMO | HCR_IMO | HCR_FMO | \
>> +#define HCR_GUEST_FLAGS (HCR_TSC | HCR_TSW | HCR_TWE | HCR_TWI | HCR_VM | \
>> +			 HCR_BSU_IS | HCR_FB | HCR_TAC | \
>> +			 HCR_AMO | HCR_IMO | HCR_FMO | \
>>  			 HCR_SWIO | HCR_TIDCP | HCR_RW)
>>  #define HCR_VIRT_EXCP_MASK (HCR_VA | HCR_VI | HCR_VF)
>>
>> @@ -242,4 +244,6 @@
>>
>>  #define ESR_EL2_EC_xABT_xFSR_EXTABT	0x10
>>
>> +#define ESR_EL2_EC_WFI_ISS_WFE	(1 << 0)
> 
> In another patch this is named as WHI_IS_WFE whereas here it is WFI_ISS_WFE, looks like typo. Anyways, what I am interested to understand is what does this macro means?

Not a typo. It decodes as:
Exception Status Register, Exception Level 2, Exception Class Wait For
Interrupt, Instruction Specific Syndrome Wait For Event.

The ARM code doesn't have such a convention, so I didn't bother. It just
reads "Hyp Status Register, Wait For Interrupt Is Wait For Event".

	M.

> Thanks
> -Bharat
> 
>> +
>>  #endif /* __ARM64_KVM_ARM_H__ */
>> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c index
>> 9beaca03..8da5606 100644
>> --- a/arch/arm64/kvm/handle_exit.c
>> +++ b/arch/arm64/kvm/handle_exit.c
>> @@ -47,21 +47,29 @@ static int handle_smc(struct kvm_vcpu *vcpu, struct kvm_run
>> *run)  }
>>
>>  /**
>> - * kvm_handle_wfi - handle a wait-for-interrupts instruction executed by a
>> guest
>> + * kvm_handle_wfx - handle a wait-for-interrupts or wait-for-event
>> + *		    instruction executed by a guest
>> + *
>>   * @vcpu:	the vcpu pointer
>>   *
>> - * Simply call kvm_vcpu_block(), which will halt execution of
>> + * WFE: Yield the CPU and come back to this vcpu when the scheduler
>> + * decides to.
>> + * WFI: Simply call kvm_vcpu_block(), which will halt execution of
>>   * world-switches and schedule other host processes until there is an
>>   * incoming IRQ or FIQ to the VM.
>>   */
>> -static int kvm_handle_wfi(struct kvm_vcpu *vcpu, struct kvm_run *run)
>> +static int kvm_handle_wfx(struct kvm_vcpu *vcpu, struct kvm_run *run)
>>  {
>> -	kvm_vcpu_block(vcpu);
>> +	if (kvm_vcpu_get_hsr(vcpu) & ESR_EL2_EC_WFI_ISS_WFE)
>> +		kvm_vcpu_on_spin(vcpu);
>> +	else
>> +		kvm_vcpu_block(vcpu);
>> +
>>  	return 1;
>>  }
>>
>>  static exit_handle_fn arm_exit_handlers[] = {
>> -	[ESR_EL2_EC_WFI]	= kvm_handle_wfi,
>> +	[ESR_EL2_EC_WFI]	= kvm_handle_wfx,
>>  	[ESR_EL2_EC_CP15_32]	= kvm_handle_cp15_32,
>>  	[ESR_EL2_EC_CP15_64]	= kvm_handle_cp15_64,
>>  	[ESR_EL2_EC_CP14_MR]	= kvm_handle_cp14_access,
>> --
>> 1.8.2.3
>>
>>
>>
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>> kvmarm at lists.cs.columbia.edu
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> 
> 
> 

-- 
Jazz is not dead. It just smells funny...