Re: [RFC PATCH 2/5] KVM: arm64: Move WFI execution to check_vcpu_requests()

[Fuad Tabba]

Hi Jean-Philippe,

On Tue, Jun 8, 2021 at 4:54 PM Jean-Philippe Brucker
<jean-philippe@xxxxxxxxxx> wrote:
>
> Prepare for WFI requests from userspace, by adding a suspend request and
> moving the WFI execution into check_vcpu_requests(), next to the
> power-off logic.
>
> vcpu->arch.mp_state, previously only RUNNABLE or STOPPED, supports an
> additional state HALTED and two new state transitions:
>
>   RUNNABLE -> HALTED    from WFI or PSCI CPU_SUSPEND (same vCPU)
>   HALTED -> RUNNABLE    vGIC IRQ, pending timer, signal
>
> There shouldn't be any functional change with this patch, even though
> the KVM_GET_MP_STATE ioctl could now in theory return
> KVM_MP_STATE_HALTED, which would break some users' mp_state support. In
> practice it should not happen because we do not return to userspace with
> HALTED state. Both WFI and PSCI CPU_SUSPEND stay in the vCPU run loop
> until the suspend request is consumed. It does feel fragile though,
> maybe we should explicitly return RUNNABLE in KVM_GET_MP_STATE in place
> of HALTED, to prevent future breakage.

It's not really a functional change, but it might introduce some
timing/scheduling changes I think.

Before your changes, the kvm_vcpu_block() would take place at the end
of the vCPU run loop, via handle_exit(). Now it takes place closer to
the beginning, after cond_resched() is called, and not if there is a
KVM_REQ_IRQ_PENDING.

If my observation is correct, would it be good to mention that?

Thanks,
/fuad



>
> Signed-off-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/kvm_host.h |  2 ++
>  arch/arm64/kvm/arm.c              | 18 ++++++++++++++-
>  arch/arm64/kvm/handle_exit.c      |  3 +--
>  arch/arm64/kvm/psci.c             | 37 +++++++++++++------------------
>  4 files changed, 35 insertions(+), 25 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 55a04f4d5919..3ca732feb9a5 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -46,6 +46,7 @@
>  #define KVM_REQ_VCPU_RESET     KVM_ARCH_REQ(2)
>  #define KVM_REQ_RECORD_STEAL   KVM_ARCH_REQ(3)
>  #define KVM_REQ_RELOAD_GICv4   KVM_ARCH_REQ(4)
> +#define KVM_REQ_SUSPEND                KVM_ARCH_REQ(5)
>
>  #define KVM_DIRTY_LOG_MANUAL_CAPS   (KVM_DIRTY_LOG_MANUAL_PROTECT_ENABLE | \
>                                      KVM_DIRTY_LOG_INITIALLY_SET)
> @@ -722,6 +723,7 @@ int kvm_arm_vcpu_arch_has_attr(struct kvm_vcpu *vcpu,
>                                struct kvm_device_attr *attr);
>  void kvm_arm_vcpu_power_off(struct kvm_vcpu *vcpu);
>  bool kvm_arm_vcpu_is_off(struct kvm_vcpu *vcpu);
> +void kvm_arm_vcpu_suspend(struct kvm_vcpu *vcpu);
>
>  /* Guest/host FPSIMD coordination helpers */
>  int kvm_arch_vcpu_run_map_fp(struct kvm_vcpu *vcpu);
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index bcc24adb9c0a..d8cbaa0373c7 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -447,6 +447,12 @@ bool kvm_arm_vcpu_is_off(struct kvm_vcpu *vcpu)
>         return vcpu->arch.mp_state == KVM_MP_STATE_STOPPED;
>  }
>
> +void kvm_arm_vcpu_suspend(struct kvm_vcpu *vcpu)
> +{
> +       vcpu->arch.mp_state = KVM_MP_STATE_HALTED;
> +       kvm_make_request(KVM_REQ_SUSPEND, vcpu);
> +}
> +
>  int kvm_arch_vcpu_ioctl_get_mpstate(struct kvm_vcpu *vcpu,
>                                     struct kvm_mp_state *mp_state)
>  {
> @@ -667,6 +673,8 @@ static int kvm_vcpu_initialized(struct kvm_vcpu *vcpu)
>
>  static void check_vcpu_requests(struct kvm_vcpu *vcpu)
>  {
> +       bool irq_pending;
> +
>         if (kvm_request_pending(vcpu)) {
>                 if (kvm_check_request(KVM_REQ_SLEEP, vcpu))
>                         vcpu_req_sleep(vcpu);
> @@ -678,7 +686,7 @@ static void check_vcpu_requests(struct kvm_vcpu *vcpu)
>                  * Clear IRQ_PENDING requests that were made to guarantee
>                  * that a VCPU sees new virtual interrupts.
>                  */
> -               kvm_check_request(KVM_REQ_IRQ_PENDING, vcpu);
> +               irq_pending = kvm_check_request(KVM_REQ_IRQ_PENDING, vcpu);
>
>                 if (kvm_check_request(KVM_REQ_RECORD_STEAL, vcpu))
>                         kvm_update_stolen_time(vcpu);
> @@ -690,6 +698,14 @@ static void check_vcpu_requests(struct kvm_vcpu *vcpu)
>                         vgic_v4_load(vcpu);
>                         preempt_enable();
>                 }
> +
> +               if (kvm_check_request(KVM_REQ_SUSPEND, vcpu)) {
> +                       if (!irq_pending) {
> +                               kvm_vcpu_block(vcpu);
> +                               kvm_clear_request(KVM_REQ_UNHALT, vcpu);
> +                       }
> +                       vcpu->arch.mp_state = KVM_MP_STATE_RUNNABLE;
> +               }
>         }
>  }
>
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index 6f48336b1d86..9717df3104cf 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -95,8 +95,7 @@ static int kvm_handle_wfx(struct kvm_vcpu *vcpu)
>         } else {
>                 trace_kvm_wfx_arm64(*vcpu_pc(vcpu), false);
>                 vcpu->stat.wfi_exit_stat++;
> -               kvm_vcpu_block(vcpu);
> -               kvm_clear_request(KVM_REQ_UNHALT, vcpu);
> +               kvm_arm_vcpu_suspend(vcpu);
>         }
>
>         kvm_incr_pc(vcpu);
> diff --git a/arch/arm64/kvm/psci.c b/arch/arm64/kvm/psci.c
> index 24b4a2265dbd..42a307ceb95f 100644
> --- a/arch/arm64/kvm/psci.c
> +++ b/arch/arm64/kvm/psci.c
> @@ -31,27 +31,6 @@ static unsigned long psci_affinity_mask(unsigned long affinity_level)
>         return 0;
>  }
>
> -static unsigned long kvm_psci_vcpu_suspend(struct kvm_vcpu *vcpu)
> -{
> -       /*
> -        * NOTE: For simplicity, we make VCPU suspend emulation to be
> -        * same-as WFI (Wait-for-interrupt) emulation.
> -        *
> -        * This means for KVM the wakeup events are interrupts and
> -        * this is consistent with intended use of StateID as described
> -        * in section 5.4.1 of PSCI v0.2 specification (ARM DEN 0022A).
> -        *
> -        * Further, we also treat power-down request to be same as
> -        * stand-by request as-per section 5.4.2 clause 3 of PSCI v0.2
> -        * specification (ARM DEN 0022A). This means all suspend states
> -        * for KVM will preserve the register state.
> -        */
> -       kvm_vcpu_block(vcpu);
> -       kvm_clear_request(KVM_REQ_UNHALT, vcpu);
> -
> -       return PSCI_RET_SUCCESS;
> -}
> -
>  static unsigned long kvm_psci_vcpu_on(struct kvm_vcpu *source_vcpu)
>  {
>         struct vcpu_reset_state *reset_state;
> @@ -227,7 +206,21 @@ static int kvm_psci_0_2_call(struct kvm_vcpu *vcpu)
>                 break;
>         case PSCI_0_2_FN_CPU_SUSPEND:
>         case PSCI_0_2_FN64_CPU_SUSPEND:
> -               val = kvm_psci_vcpu_suspend(vcpu);
> +               /*
> +                * NOTE: For simplicity, we make VCPU suspend emulation to be
> +                * same-as WFI (Wait-for-interrupt) emulation.
> +                *
> +                * This means for KVM the wakeup events are interrupts and this
> +                * is consistent with intended use of StateID as described in
> +                * section 5.4.1 of PSCI v0.2 specification (ARM DEN 0022A).
> +                *
> +                * Further, we also treat power-down request to be same as
> +                * stand-by request as-per section 5.4.2 clause 3 of PSCI v0.2
> +                * specification (ARM DEN 0022A). This means all suspend states
> +                * for KVM will preserve the register state.
> +                */
> +               kvm_arm_vcpu_suspend(vcpu);
> +               val = PSCI_RET_SUCCESS;
>                 break;
>         case PSCI_0_2_FN_CPU_OFF:
>                 kvm_arm_vcpu_power_off(vcpu);
> --
> 2.31.1
>
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