On 07/10/13 16:52, Bhushan Bharat-R65777 wrote:
>
>
>> -----Original Message-----
>> From: Marc Zyngier [mailto:[email protected]]
>> Sent: Monday, October 07, 2013 9:11 PM
>> To: [email protected]; [email protected];
>> [email protected]
>> 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 protected]>
>> ---
>> 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
>>
>>
>>
>> _______________________________________________
>> kvmarm mailing list
>> [email protected]
>> https://lists.cs.columbia.edu/cucslists/listinfo/kvmarm
>
>
>
--
Jazz is not dead. It just smells funny...
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