[RFC 13/33] KVM: Allow polling vCPUs for events

[Nicolas Saenz Julienne <nsaenz@amazon.com>]

A number of use cases have surfaced where it'd be beneficial to have a
vCPU stop its execution in user-space, as opposed to having it sleep
in-kernel. Be it in order to make better use of the pCPU's time while
the vCPU is halted, or to implement security features like Hyper-V's
VSM.

A problem with this approach is that user-space has no way of knowing
whether the vCPU has pending events (interrupts, timers, etc...), so we
need a new interface to query if they are. poll() turned out to be a
very good fit.

So enable polling vCPUs. The poll() interface considers a vCPU has a
pending event if it didn't enter the guest since being kicked by an
event source (being kicked forces a guest exit). Kicking a vCPU that has
pollers wakes up the polling threads.

NOTES:
 - There is a race between the 'vcpu->kicked' check in the polling
   thread and the vCPU thread re-entering the guest. This hardly affects
   the use-cases stated above, but needs to be fixed.

 - This was tested alongside a WIP Hyper-V Virtual Trust Level
   implementation which makes ample use of the poll() interface.

Signed-off-by: Nicolas Saenz Julienne <nsaenz@amazon.com>
---
 arch/x86/kvm/x86.c       |  2 ++
 include/linux/kvm_host.h |  2 ++
 virt/kvm/kvm_main.c      | 30 ++++++++++++++++++++++++++++++
 3 files changed, 34 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 57f9c58e1e32..bf4891bc044e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10788,6 +10788,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 		goto cancel_injection;
 	}
 
+	WRITE_ONCE(vcpu->kicked, false);
+
 	if (req_immediate_exit) {
 		kvm_make_request(KVM_REQ_EVENT, vcpu);
 		static_call(kvm_x86_request_immediate_exit)(vcpu);
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 687589ce9f63..71e1e8cf8936 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -336,6 +336,7 @@ struct kvm_vcpu {
 #endif
 	int mode;
 	u64 requests;
+	bool kicked;
 	unsigned long guest_debug;
 
 	struct mutex mutex;
@@ -395,6 +396,7 @@ struct kvm_vcpu {
 	 */
 	struct kvm_memory_slot *last_used_slot;
 	u64 last_used_slot_gen;
+	wait_queue_head_t wqh;
 };
 
 /*
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index ad9aab898a0c..fde004a0ac46 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -497,12 +497,14 @@ static void kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id)
 	kvm_vcpu_set_dy_eligible(vcpu, false);
 	vcpu->preempted = false;
 	vcpu->ready = false;
+	vcpu->kicked = false;
 	preempt_notifier_init(&vcpu->preempt_notifier, &kvm_preempt_ops);
 	vcpu->last_used_slot = NULL;
 
 	/* Fill the stats id string for the vcpu */
 	snprintf(vcpu->stats_id, sizeof(vcpu->stats_id), "kvm-%d/vcpu-%d",
 		 task_pid_nr(current), id);
+	init_waitqueue_head(&vcpu->wqh);
 }
 
 static void kvm_vcpu_destroy(struct kvm_vcpu *vcpu)
@@ -3970,6 +3972,10 @@ void kvm_vcpu_kick(struct kvm_vcpu *vcpu)
 		if (cpu != me && (unsigned)cpu < nr_cpu_ids && cpu_online(cpu))
 			smp_send_reschedule(cpu);
 	}
+
+	if (!cmpxchg(&vcpu->kicked, false, true))
+		wake_up_interruptible(&vcpu->wqh);
+
 out:
 	put_cpu();
 }
@@ -4174,6 +4180,29 @@ static int kvm_vcpu_mmap(struct file *file, struct vm_area_struct *vma)
 	return 0;
 }
 
+static __poll_t kvm_vcpu_poll(struct file *file, poll_table *wait)
+{
+	struct kvm_vcpu *vcpu = file->private_data;
+
+	poll_wait(file, &vcpu->wqh, wait);
+
+	/*
+	 * Make sure we read vcpu->kicked after adding the vcpu into
+	 * the waitqueue list. Otherwise we might have the following race:
+	 *
+	 *   READ_ONCE(vcpu->kicked)
+	 *					cmpxchg(&vcpu->kicked, false, true))
+	 *					wake_up_interruptible(&vcpu->wqh)
+	 *   list_add_tail(wait, &vcpu->wqh)
+	 */
+	smp_mb();
+	if (READ_ONCE(vcpu->kicked)) {
+		return EPOLLIN;
+	}
+
+	return 0;
+}
+
 static int kvm_vcpu_release(struct inode *inode, struct file *filp)
 {
 	struct kvm_vcpu *vcpu = filp->private_data;
@@ -4186,6 +4215,7 @@ static const struct file_operations kvm_vcpu_fops = {
 	.release        = kvm_vcpu_release,
 	.unlocked_ioctl = kvm_vcpu_ioctl,
 	.mmap           = kvm_vcpu_mmap,
+	.poll		= kvm_vcpu_poll,
 	.llseek		= noop_llseek,
 	KVM_COMPAT(kvm_vcpu_compat_ioctl),
 };
-- 
2.40.1