[PATCH v6 06/12] KVM: arm64: Add support for userspace to suspend a vCPU

From: Oliver Upton
Date: Tue May 03 2022 - 23:25:30 EST

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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@xxxxxxxxxx>
---
Documentation/virt/kvm/api.rst | 37 ++++++++++++++++++++--
arch/arm64/include/asm/kvm_host.h | 1 +
arch/arm64/kvm/arm.c | 51 +++++++++++++++++++++++++++++++
include/uapi/linux/kvm.h | 2 ++
4 files changed, 89 insertions(+), 2 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 4a900cdbc62e..46ca84600dca 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;
__u32 ndata;
__u64 data[16];
@@ -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.

If KVM_CAP_SYSTEM_EVENT_DATA is present, the 'data' field can contain
architecture specific information for the system-level event. Only
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..abd32a84ed7a 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 +663,39 @@ 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);
+
+ /*
+ * Check to make sure the vCPU is actually runnable. If so, exit to
+ * userspace informing it of the wakeup condition.
+ */
+ if (kvm_arch_vcpu_runnable(vcpu)) {
+ memset(&vcpu->run->system_event, 0, sizeof(vcpu->run->system_event));
+ vcpu->run->system_event.type = KVM_SYSTEM_EVENT_WAKEUP;
+ vcpu->run->exit_reason = KVM_EXIT_SYSTEM_EVENT;
+ return 0;
+ }
+
+ /*
+ * Otherwise, we were unblocked to process a different event, such as a
+ * pending signal. Return 1 and allow kvm_arch_vcpu_ioctl_run() to
+ * process the event.
+ */
+ return 1;
+}
+
/**
* check_vcpu_requests - check and handle pending vCPU requests
* @vcpu: the VCPU pointer
@@ -686,6 +734,9 @@ 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);
}

return 1;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 6a184d260c7f..7f72fb7b05f2 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -444,6 +444,7 @@ struct kvm_run {
#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;
__u32 ndata;
union {
@@ -646,6 +647,7 @@ struct kvm_vapic_addr {
#define KVM_MP_STATE_OPERATING 7
#define KVM_MP_STATE_LOAD 8
#define KVM_MP_STATE_AP_RESET_HOLD 9
+#define KVM_MP_STATE_SUSPENDED 10

struct kvm_mp_state {
__u32 mp_state;
--
2.36.0.464.gb9c8b46e94-goog

Next message: Oliver Upton: "[PATCH v6 07/12] KVM: arm64: Implement PSCI SYSTEM_SUSPEND"
Previous message: Oliver Upton: "[PATCH v6 04/12] KVM: arm64: Rename the KVM_REQ_SLEEP handler"
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Next in thread: Oliver Upton: "[PATCH v6 07/12] KVM: arm64: Implement PSCI SYSTEM_SUSPEND"
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