[RFC PATCH 3/5] KVM: arm64: Allow userspace to request WFI

[Fuad Tabba]

Hi Jean-Philippe,

On Tue, Jun 8, 2021 at 4:54 PM Jean-Philippe Brucker
<jean-philippe at linaro.org> wrote:
>
> To help userspace implement PSCI CPU_SUSPEND, allow setting the "HALTED"
> MP state to request a WFI before returning to the guest.
>
> Userspace won't obtain a HALTED mp_state from a KVM_GET_MP_STATE call
> unless they set it themselves. When set by KVM, to handle wfi or
> CPU_SUSPEND, it is consumed before returning to userspace.
>
> Signed-off-by: Jean-Philippe Brucker <jean-philippe at linaro.org>
> ---
>  Documentation/virt/kvm/api.rst | 15 +++++++++------
>  include/uapi/linux/kvm.h       |  1 +
>  arch/arm64/kvm/arm.c           | 11 ++++++++++-
>  3 files changed, 20 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 7fcb2fd38f42..e4fe7fb60d5d 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -1416,8 +1416,8 @@ Possible values are:
>                                   which has not yet received an INIT signal [x86]
>     KVM_MP_STATE_INIT_RECEIVED    the vcpu has received an INIT signal, and is
>                                   now ready for a SIPI [x86]
> -   KVM_MP_STATE_HALTED           the vcpu has executed a HLT instruction and
> -                                 is waiting for an interrupt [x86]
> +   KVM_MP_STATE_HALTED           the vcpu has executed a HLT/WFI instruction
> +                                 and is waiting for an interrupt [x86,arm64]

Considering that arm64 has a HLT instruction (for debugging), which is
very different from the x86 one, would it be good to clarify that in
the comment. e.g.,

"the vcpu has executed a HLT(x86)/WFI(arm64) instruction"?

Thanks,
/fuad


>     KVM_MP_STATE_SIPI_RECEIVED    the vcpu has just received a SIPI (vector
>                                   accessible via KVM_GET_VCPU_EVENTS) [x86]
>     KVM_MP_STATE_STOPPED          the vcpu is stopped [s390,arm/arm64]
> @@ -1435,8 +1435,9 @@ these architectures.
>  For arm/arm64:
>  ^^^^^^^^^^^^^^
>
> -The only states that are valid are KVM_MP_STATE_STOPPED and
> -KVM_MP_STATE_RUNNABLE which reflect if the vcpu is paused or not.
> +Valid states are KVM_MP_STATE_STOPPED and KVM_MP_STATE_RUNNABLE which reflect
> +if the vcpu is paused or not. If KVM_CAP_ARM_MP_HALTED is present, state
> +KVM_MP_STATE_HALTED is also valid.
>
>  4.39 KVM_SET_MP_STATE
>  ---------------------
> @@ -1457,8 +1458,10 @@ these architectures.
>  For arm/arm64:
>  ^^^^^^^^^^^^^^
>
> -The only states that are valid are KVM_MP_STATE_STOPPED and
> -KVM_MP_STATE_RUNNABLE which reflect if the vcpu should be paused or not.
> +Valid states are KVM_MP_STATE_STOPPED and KVM_MP_STATE_RUNNABLE which reflect
> +if the vcpu should be paused or not. If KVM_CAP_ARM_MP_HALTED is present,
> +KVM_MP_STATE_HALTED can be set, to wait for interrupts targeted at the vcpu
> +before running it.
>
>  4.40 KVM_SET_IDENTITY_MAP_ADDR
>  ------------------------------
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 79d9c44d1ad7..06ba64c49737 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1083,6 +1083,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_SGX_ATTRIBUTE 196
>  #define KVM_CAP_VM_COPY_ENC_CONTEXT_FROM 197
>  #define KVM_CAP_PTP_KVM 198
> +#define KVM_CAP_ARM_MP_HALTED 199
>
>  #ifdef KVM_CAP_IRQ_ROUTING
>
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index d8cbaa0373c7..d6ad977fea5f 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -207,6 +207,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>         case KVM_CAP_SET_GUEST_DEBUG:
>         case KVM_CAP_VCPU_ATTRIBUTES:
>         case KVM_CAP_PTP_KVM:
> +       case KVM_CAP_ARM_MP_HALTED:
>                 r = 1;
>                 break;
>         case KVM_CAP_SET_GUEST_DEBUG2:
> @@ -469,6 +470,9 @@ int kvm_arch_vcpu_ioctl_set_mpstate(struct kvm_vcpu *vcpu,
>         case KVM_MP_STATE_RUNNABLE:
>                 vcpu->arch.mp_state = KVM_MP_STATE_RUNNABLE;
>                 break;
> +       case KVM_MP_STATE_HALTED:
> +               kvm_arm_vcpu_suspend(vcpu);
> +               break;
>         case KVM_MP_STATE_STOPPED:
>                 kvm_arm_vcpu_power_off(vcpu);
>                 break;
> @@ -699,7 +703,12 @@ static void check_vcpu_requests(struct kvm_vcpu *vcpu)
>                         preempt_enable();
>                 }
>
> -               if (kvm_check_request(KVM_REQ_SUSPEND, vcpu)) {
> +               /*
> +                * Check mp_state again in case userspace changed their mind
> +                * after requesting suspend.
> +                */
> +               if (kvm_check_request(KVM_REQ_SUSPEND, vcpu) &&
> +                   vcpu->arch.mp_state == KVM_MP_STATE_HALTED) {
>                         if (!irq_pending) {
>                                 kvm_vcpu_block(vcpu);
>                                 kvm_clear_request(KVM_REQ_UNHALT, vcpu);
> --
> 2.31.1
>
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