Re: [PATCH v4 09/15] KVM: arm64: Add support for userspace to suspend a vCPU

[Oliver Upton <oupton@google.com>]

On Mon, Mar 21, 2022 at 11:19:33PM -0700, Reiji Watanabe wrote:
> Hi Oliver,
> 
> On Fri, Mar 11, 2022 at 9:41 AM Oliver Upton <oupton@google.com> wrote:
> >
> > Introduce a new MP state, KVM_MP_STATE_SUSPENDED, which indicates a vCPU
> > is in a suspended state. In the suspended state the vCPU will block
> > until a wakeup event (pending interrupt) is recognized.
> >
> > Add a new system event type, KVM_SYSTEM_EVENT_WAKEUP, to indicate to
> > userspace that KVM has recognized one such wakeup event. It is the
> > responsibility of userspace to then make the vCPU runnable, or leave it
> > suspended until the next wakeup event.
> >
> > Signed-off-by: Oliver Upton <oupton@google.com>
> > ---
> >  Documentation/virt/kvm/api.rst    | 37 +++++++++++++++++++++++++++++--
> >  arch/arm64/include/asm/kvm_host.h |  1 +
> >  arch/arm64/kvm/arm.c              | 35 +++++++++++++++++++++++++++++
> >  include/uapi/linux/kvm.h          |  2 ++
> >  4 files changed, 73 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index 5625c08b4a0e..426bcdc1216d 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -1482,14 +1482,43 @@ Possible values are:
> >                                   [s390]
> >     KVM_MP_STATE_LOAD             the vcpu is in a special load/startup state
> >                                   [s390]
> > +   KVM_MP_STATE_SUSPENDED        the vcpu is in a suspend state and is waiting
> > +                                 for a wakeup event [arm64]
> >     ==========================    ===============================================
> >
> >  On x86, this ioctl is only useful after KVM_CREATE_IRQCHIP. Without an
> >  in-kernel irqchip, the multiprocessing state must be maintained by userspace on
> >  these architectures.
> >
> > -For arm64/riscv:
> > -^^^^^^^^^^^^^^^^
> > +For arm64:
> > +^^^^^^^^^^
> > +
> > +If a vCPU is in the KVM_MP_STATE_SUSPENDED state, KVM will emulate the
> > +architectural execution of a WFI instruction.
> > +
> > +If a wakeup event is recognized, KVM will exit to userspace with a
> > +KVM_SYSTEM_EVENT exit, where the event type is KVM_SYSTEM_EVENT_WAKEUP. If
> > +userspace wants to honor the wakeup, it must set the vCPU's MP state to
> > +KVM_MP_STATE_RUNNABLE. If it does not, KVM will continue to await a wakeup
> > +event in subsequent calls to KVM_RUN.
> > +
> > +.. warning::
> > +
> > +     If userspace intends to keep the vCPU in a SUSPENDED state, it is
> > +     strongly recommended that userspace take action to suppress the
> > +     wakeup event (such as masking an interrupt). Otherwise, subsequent
> > +     calls to KVM_RUN will immediately exit with a KVM_SYSTEM_EVENT_WAKEUP
> > +     event and inadvertently waste CPU cycles.
> > +
> > +     Additionally, if userspace takes action to suppress a wakeup event,
> > +     it is strongly recommended that it also restore the vCPU to its
> 
> Nit: s/restore/restores/ ?
> 
> 
> > +     original state when the vCPU is made RUNNABLE again. For example,
> > +     if userspace masked a pending interrupt to suppress the wakeup,
> > +     the interrupt should be unmasked before returning control to the
> > +     guest.
> > +
> > +For riscv:
> > +^^^^^^^^^^
> >
> >  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.
> > @@ -5914,6 +5943,7 @@ should put the acknowledged interrupt vector into the 'epr' field.
> >    #define KVM_SYSTEM_EVENT_SHUTDOWN       1
> >    #define KVM_SYSTEM_EVENT_RESET          2
> >    #define KVM_SYSTEM_EVENT_CRASH          3
> > +  #define KVM_SYSTEM_EVENT_WAKEUP         4
> >                         __u32 type;
> >                         __u64 flags;
> >                 } system_event;
> > @@ -5938,6 +5968,9 @@ Valid values for 'type' are:
> >     has requested a crash condition maintenance. Userspace can choose
> >     to ignore the request, or to gather VM memory core dump and/or
> >     reset/shutdown of the VM.
> > + - KVM_SYSTEM_EVENT_WAKEUP -- the guest is in a suspended state and KVM
> 
> Nit: Shouldn't 'the guest' be 'the vcpu' ?
> 
> > +   has recognized a wakeup event. Userspace may honor this event by marking
> > +   the exiting vCPU as runnable, or deny it and call KVM_RUN again.
> >
> >  Valid flags are:
> >
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index da58eb96d2a8..899f2c0b4c7b 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -46,6 +46,7 @@
> >  #define KVM_REQ_RECORD_STEAL   KVM_ARCH_REQ(3)
> >  #define KVM_REQ_RELOAD_GICv4   KVM_ARCH_REQ(4)
> >  #define KVM_REQ_RELOAD_PMU     KVM_ARCH_REQ(5)
> > +#define KVM_REQ_SUSPEND                KVM_ARCH_REQ(6)
> >
> >  #define KVM_DIRTY_LOG_MANUAL_CAPS   (KVM_DIRTY_LOG_MANUAL_PROTECT_ENABLE | \
> >                                      KVM_DIRTY_LOG_INITIALLY_SET)
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 8eed0556ccaa..b94efa05d869 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -444,6 +444,18 @@ bool kvm_arm_vcpu_stopped(struct kvm_vcpu *vcpu)
> >         return vcpu->arch.mp_state.mp_state == KVM_MP_STATE_STOPPED;
> >  }
> >
> > +static void kvm_arm_vcpu_suspend(struct kvm_vcpu *vcpu)
> > +{
> > +       vcpu->arch.mp_state.mp_state = KVM_MP_STATE_SUSPENDED;
> > +       kvm_make_request(KVM_REQ_SUSPEND, vcpu);
> > +       kvm_vcpu_kick(vcpu);
> > +}
> > +
> > +static bool kvm_arm_vcpu_suspended(struct kvm_vcpu *vcpu)
> > +{
> > +       return vcpu->arch.mp_state.mp_state == KVM_MP_STATE_SUSPENDED;
> > +}
> > +
> >  int kvm_arch_vcpu_ioctl_get_mpstate(struct kvm_vcpu *vcpu,
> >                                     struct kvm_mp_state *mp_state)
> >  {
> > @@ -464,6 +476,9 @@ int kvm_arch_vcpu_ioctl_set_mpstate(struct kvm_vcpu *vcpu,
> >         case KVM_MP_STATE_STOPPED:
> >                 kvm_arm_vcpu_power_off(vcpu);
> >                 break;
> > +       case KVM_MP_STATE_SUSPENDED:
> > +               kvm_arm_vcpu_suspend(vcpu);
> > +               break;
> >         default:
> >                 ret = -EINVAL;
> >         }
> > @@ -648,6 +663,23 @@ void kvm_vcpu_wfi(struct kvm_vcpu *vcpu)
> >         preempt_enable();
> >  }
> >
> > +static int kvm_vcpu_suspend(struct kvm_vcpu *vcpu)
> > +{
> > +       if (!kvm_arm_vcpu_suspended(vcpu))
> > +               return 1;
> > +
> > +       kvm_vcpu_wfi(vcpu);
> > +
> > +       /*
> > +        * The suspend state is sticky; we do not leave it until userspace
> > +        * explicitly marks the vCPU as runnable. Request that we suspend again
> > +        * later.
> > +        */
> > +       kvm_make_request(KVM_REQ_SUSPEND, vcpu);
> > +       kvm_vcpu_set_system_event_exit(vcpu, KVM_SYSTEM_EVENT_WAKEUP, 0);
> > +       return 0;
> > +}
> > +
> >  /**
> >   * check_vcpu_requests - check and handle pending vCPU requests
> >   * @vcpu:      the VCPU pointer
> > @@ -686,6 +718,9 @@ static int check_vcpu_requests(struct kvm_vcpu *vcpu)
> >                 if (kvm_check_request(KVM_REQ_RELOAD_PMU, vcpu))
> >                         kvm_pmu_handle_pmcr(vcpu,
> >                                             __vcpu_sys_reg(vcpu, PMCR_EL0));
> > +
> > +               if (kvm_check_request(KVM_REQ_SUSPEND, vcpu))
> > +                       return kvm_vcpu_suspend(vcpu);
> 
> It appears that one of the cases that kvm_vcpu_suspend() returns
> is when a pending signal is detected, and the exit reason will be
> KVM_EXIT_SYSTEM_EVENT in this case.  On the other hand, when a
> pending signal is detected earlier in xfer_to_guest_mode_handle_work(),
> KVM_RUN returns -EINTR even if the vCPU is in KVM_MP_STATE_SUSPENDED
> state. Shouldn't those behaviors be consistent ? (Perhaps -EINTR?)

Great catch!

I should probably check that the vCPU is actually runnable with
kvm_arch_vcpu_runnable() before setting up a system event exit. That is
after all what the documentation of this whole API says it does, right?
:)

If the vCPU thread were poked for any other reason this should return 1
and let the kvm_vcpu_exit_request()/xfer_to_guest_mode_handle_work()
pairing clean up anything else.

Appreciate the review, other comments noted and will be addressed.

--
Thanks,
Oliver
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