[PATCH v5 07/13] KVM: arm64: Add support for userspace to suspend a vCPU
Oliver Upton
oupton at google.com
Wed Apr 20 20:23:53 PDT 2022
Hi Reiji,
On Wed, Apr 20, 2022 at 8:13 PM Reiji Watanabe <reijiw at google.com> wrote:
>
> Hi Oliver,
>
> On Sat, Apr 9, 2022 at 11:46 AM Oliver Upton <oupton at 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 at google.com>
> > ---
> > Documentation/virt/kvm/api.rst | 37 +++++++++++++++++++++--
> > arch/arm64/include/asm/kvm_host.h | 1 +
> > arch/arm64/kvm/arm.c | 49 +++++++++++++++++++++++++++++++
> > include/uapi/linux/kvm.h | 2 ++
> > 4 files changed, 87 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index d13fa6600467..d104e34ad703 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -1476,14 +1476,43 @@ 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 restores the vCPU to its
> > + 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.
> > @@ -5985,6 +6014,7 @@ 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;
> > @@ -6009,6 +6039,9 @@ 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 exiting vCPU is in a suspended state and
> > + KVM 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 f3f93d48e21a..46027b9b80ca 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 efe54aba5cce..e9641b86d375 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 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);
>
> Considering the patch 8 will remove the call to kvm_vcpu_kick()
> (BTW, I wonder why you wanted to make that change in the patch-8
> instead of the patch-7),
Squashed the diff into the wrong patch! Marc pointed out this is of
course cargo-culted as I was following the pattern laid down by
KVM_REQ_SLEEP :)
> it looks like we could use the mp_state
> KVM_MP_STATE_SUSPENDED instead of using KVM_REQ_SUSPEND.
> What is the reason why you prefer to introduce KVM_REQ_SUSPEND
> rather than simply using KVM_MP_STATE_SUSPENDED ?
I was trying to avoid any heavy refactoring in adding new
functionality here, as we handle KVM_MP_STATE_STOPPED similarly (make
a request). ARM is definitely a bit different than x86 in the way that
we handle the MP states, as x86 doesn't bounce through vCPU requests
to do it and instead directly checks the mp_state value.
Do you think it's fair to defer on repainting to a later series? We
probably will need to touch up the main run loop quite a lot along the
way.
--
Thanks,
Oliver
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