On Mon, Jul 02, 2018 at 05:46:21PM +0100, Marc Zyngier wrote:
> Trapping blocking WFE is extremely beneficial in situations where
> the system is oversubscribed, as it allows another thread to run
> while being blocked. In a non-oversubscribed environment, this is
> the complete opposite, and trapping WFE is just unnecessary overhead.
>
> Let's only enable WFE trapping if the CPU has more than a single task
> to run (that is, more than just the vcpu thread).
>
> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
Reviewed-by: Christoffer Dall <christoffer.dall@xxxxxxx>
> ---
> This small patchlet seem to have some interesting effects in my limited
> testing. Installing 100 concurrent Debian VMs results results in about
> 90 million WFE exits without this patch, and only 7.6 millions with it
> (TX1, 96 cores, 2 vcpu guests). I haven't measured any significant
> performance variation in that process.
>
> I'd welcome any benchmarking result from people with more interesting
> workloads than mine so that I could make my mind about it.
>
> arch/arm/include/asm/kvm_emulate.h | 10 ++++++++++
> arch/arm64/include/asm/kvm_emulate.h | 10 ++++++++++
> virt/kvm/arm/arm.c | 6 ++++++
> 3 files changed, 26 insertions(+)
>
> diff --git a/arch/arm/include/asm/kvm_emulate.h b/arch/arm/include/asm/kvm_emulate.h
> index 6493bd479ddc..b50fe8380868 100644
> --- a/arch/arm/include/asm/kvm_emulate.h
> +++ b/arch/arm/include/asm/kvm_emulate.h
> @@ -107,6 +107,16 @@ static inline unsigned long *vcpu_hcr(const struct kvm_vcpu *vcpu)
> return (unsigned long *)&vcpu->arch.hcr;
> }
>
> +static inline void vcpu_clear_wfe_traps(struct kvm_vcpu *vcpu)
> +{
> + vcpu->arch.hcr &= ~HCR_TWE;
> +}
> +
> +static inline void vcpu_set_wfe_traps(struct kvm_vcpu *vcpu)
> +{
> + vcpu->arch.hcr |= HCR_TWE;
> +}
> +
> static inline bool vcpu_mode_is_32bit(const struct kvm_vcpu *vcpu)
> {
> return 1;
> diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> index 1dab3a984608..a71e5af816a9 100644
> --- a/arch/arm64/include/asm/kvm_emulate.h
> +++ b/arch/arm64/include/asm/kvm_emulate.h
> @@ -81,6 +81,16 @@ static inline unsigned long *vcpu_hcr(struct kvm_vcpu *vcpu)
> return (unsigned long *)&vcpu->arch.hcr_el2;
> }
>
> +static inline void vcpu_clear_wfe_traps(struct kvm_vcpu *vcpu)
> +{
> + vcpu->arch.hcr_el2 &= ~HCR_TWE;
> +}
> +
> +static inline void vcpu_set_wfe_traps(struct kvm_vcpu *vcpu)
> +{
> + vcpu->arch.hcr_el2 |= HCR_TWE;
> +}
> +
> static inline void vcpu_set_vsesr(struct kvm_vcpu *vcpu, u64 vsesr)
> {
> vcpu->arch.vsesr_el2 = vsesr;
> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> index 04e554cae3a2..8e66b89a3db2 100644
> --- a/virt/kvm/arm/arm.c
> +++ b/virt/kvm/arm/arm.c
> @@ -30,6 +30,7 @@
> #include <linux/kvm.h>
> #include <linux/kvm_irqfd.h>
> #include <linux/irqbypass.h>
> +#include <linux/sched/stat.h>
> #include <trace/events/kvm.h>
> #include <kvm/arm_pmu.h>
> #include <kvm/arm_psci.h>
> @@ -380,6 +381,11 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> kvm_timer_vcpu_load(vcpu);
> kvm_vcpu_load_sysregs(vcpu);
> kvm_arch_vcpu_load_fp(vcpu);
> +
> + if (single_task_running())
> + vcpu_clear_wfe_traps(vcpu);
> + else
> + vcpu_set_wfe_traps(vcpu);
> }
>
> void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> --
> 2.17.1
>
