On Tue, Oct 08, 2013 at 06:38:14PM +0100, Marc Zyngier wrote:
> 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. Also, using
> CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT slightly improves the performance
> when the VM is severely overcommited.
>
> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> ---
> arch/arm64/include/asm/kvm_arm.h | 8 ++++++--
> arch/arm64/kvm/Kconfig | 1 +
> arch/arm64/kvm/handle_exit.c | 18 +++++++++++++-----
> 3 files changed, 20 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)
> +
> #endif /* __ARM64_KVM_ARM_H__ */
> diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
> index 21e9082..4480ab3 100644
> --- a/arch/arm64/kvm/Kconfig
> +++ b/arch/arm64/kvm/Kconfig
> @@ -21,6 +21,7 @@ config KVM
> select MMU_NOTIFIER
> select PREEMPT_NOTIFIERS
> select ANON_INODES
> + select HAVE_KVM_CPU_RELAX_INTERCEPT
> select KVM_MMIO
> select KVM_ARM_HOST
> select KVM_ARM_VGIC
> 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
>
ack
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