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@xxxxxxxxxx>
---
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
