[RFC] KVM: x86: Allow userspace exit on HLT and MWAIT, else yield on MWAIT

[David Woodhouse]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

The VMM may have work to do on behalf of the guest, and it's often
desirable to use the cycles when the vCPUS are idle.

When the vCPU uses HLT this works out OK because the VMM can run its
tasks in a separate thread which gets scheduled when the in-kernel
emulation of HLT schedules away. It isn't perfect, because it doesn't
easily allow for handling both low-priority maintenance tasks when the
VMM wants to wait until the vCPU is idle, and also for higher priority
tasks where the VMM does want to preempt the vCPU. It can also lead to
noisy neighbour effects, when a host has isn't necessarily sized to
expect any given VMM to suddenly be contending for many *more* pCPUs
than it has vCPUs. 

In addition, there are times when we need to expose MWAIT to a guest
for compatibility with a previous environment. And MWAIT is much harder
because it's very hard to emulate properly.

There were attempts at doing so based on marking the target page read-
only in MONITOR and triggering the wake when it takes a minor fault,
but so far they haven't led to a working solution:
https://www.contrib.andrew.cmu.edu/~somlo/OSXKVM/mwait.html

So when a guest executes MWAIT, either we've disabled exit-on-mwait and
the guest actually sits in non-root mode hogging the pCPU, or if we do
enable exit-on-mwait the kernel just treats it as a NOP and bounces
right back into the guest to busy-wait round its idle loop.

For a start, we can stick a yield() into that busy-loop. The yield()
has fairly poorly defined semantics, but it's better than *nothing* and
does allow a VMM's thread-based I/O and maintenance tasks to run a
*little* better.

Better still, we can bounce all the way out to *userspace* on an MWAIT
exit, and let the VMM perform some of its pending work right there and
then in the vCPU thread before re-entering the vCPU. That's much nicer
than yield(). The vCPU is still runnable, since we still don't have a
*real* emulation of MWAIT, so the vCPU thread can do a *little* bit of
work and then go back into the vCPU for another turn around the loop.

And if we're going to do that kind of task processing for MWAIT-idle
guests directly from the vCPU thread, it's neater to do it for HLT-idle
guests that way too.

For HLT, the vCPU *isn't* runnable; it'll be in KVM_MP_STATE_HALTED.
The VMM can poll the mp_state and know when the vCPU should be run
again. But not poll(), although we might want to hook up something like
that (or just a signal or eventfd) for other reasons for VSM anyway.
The VMM can also just do some work and then re-enter the vCPU without
the corresponding bit set in the kvm_run struct.

So, er, what does this patch do? Add a capability, define two bits for
exiting to userspace on HLT or MWAIT — in the kvm_run struct rather
than needing a separate ioctl to turn them on or off, so that the VMM
can make the decision each time it enters the vCPU. Hook it up to
(ab?)use the existing KVM_EXIT_HLT which was previously only used when
the local APIC was emulated in userspace, and add a new KVM_EXIT_MWAIT.

Fairly much untested.

If this approach seems reasonable, of course I'll add test cases and
proper documentation before posting it for real. This is the proof of
concept before we even put it through testing to see what performance
we get out of it especially for those obnoxious MWAIT-enabled guests.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a6582c1fd8b9..8f931539114a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2128,9 +2128,23 @@ static int kvm_emulate_monitor_mwait(struct kvm_vcpu *vcpu, const char *insn)
 	pr_warn_once("%s instruction emulated as NOP!\n", insn);
 	return kvm_emulate_as_nop(vcpu);
 }
+
 int kvm_emulate_mwait(struct kvm_vcpu *vcpu)
 {
-	return kvm_emulate_monitor_mwait(vcpu, "MWAIT");
+	int ret = kvm_emulate_monitor_mwait(vcpu, "MWAIT");
+
+	if (ret && kvm_userspace_exit(vcpu, KVM_EXIT_MWAIT)) {
+		vcpu->run->exit_reason = KVM_EXIT_MWAIT;
+		ret = 0;
+	} else {
+		/*
+		 * Calling yield() has poorly defined semantics, but the
+		 * guest is in a busy loop and it's the best we can do
+		 * without a full emulation of MONITOR/MWAIT.
+		 */
+		yield();
+	}
+	return ret;
 }
 EXPORT_SYMBOL_GPL(kvm_emulate_mwait);
 
@@ -4554,6 +4568,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 				r |= KVM_X86_DISABLE_EXITS_MWAIT;
 		}
 		break;
+	case KVM_CAP_X86_USERSPACE_EXITS:
+		r = KVM_X86_USERSPACE_VALID_EXITS;
+		break;
 	case KVM_CAP_X86_SMM:
 		if (!IS_ENABLED(CONFIG_KVM_SMM))
 			break;
@@ -9643,11 +9660,11 @@ static int __kvm_emulate_halt(struct kvm_vcpu *vcpu, int state, int reason)
 	++vcpu->stat.halt_exits;
 	if (lapic_in_kernel(vcpu)) {
 		vcpu->arch.mp_state = state;
-		return 1;
-	} else {
-		vcpu->run->exit_reason = reason;
-		return 0;
+		if (!kvm_userspace_exit(vcpu, reason))
+			return 1;
 	}
+	vcpu->run->exit_reason = reason;
+	return 0;
 }
 
 int kvm_emulate_halt_noskip(struct kvm_vcpu *vcpu)
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 1e7be1f6ab29..ce10a809151c 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -430,6 +430,19 @@ static inline bool kvm_notify_vmexit_enabled(struct kvm *kvm)
 	return kvm->arch.notify_vmexit_flags & KVM_X86_NOTIFY_VMEXIT_ENABLED;
 }
 
+static inline bool kvm_userspace_exit(struct kvm_vcpu *vcpu, int reason)
+{
+	if (reason == KVM_EXIT_HLT &&
+	    (vcpu->run->userspace_exits & KVM_X86_USERSPACE_EXIT_HLT))
+		return true;
+
+	if (reason == KVM_EXIT_MWAIT &&
+	    (vcpu->run->userspace_exits & KVM_X86_USERSPACE_EXIT_MWAIT))
+		return true;
+
+	return false;
+}
+
 enum kvm_intr_type {
 	/* Values are arbitrary, but must be non-zero. */
 	KVM_HANDLING_IRQ = 1,
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 13065dd96132..43d94d49fc24 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -264,6 +264,7 @@ struct kvm_xen_exit {
 #define KVM_EXIT_RISCV_SBI        35
 #define KVM_EXIT_RISCV_CSR        36
 #define KVM_EXIT_NOTIFY           37
+#define KVM_EXIT_MWAIT            38
 
 /* For KVM_EXIT_INTERNAL_ERROR */
 /* Emulate instruction failed. */
@@ -283,7 +284,8 @@ struct kvm_run {
 	/* in */
 	__u8 request_interrupt_window;
 	__u8 immediate_exit;
-	__u8 padding1[6];
+	__u8 userspace_exits;
+	__u8 padding1[5];
 
 	/* out */
 	__u32 exit_reason;
@@ -841,6 +843,11 @@ struct kvm_ioeventfd {
                                               KVM_X86_DISABLE_EXITS_PAUSE | \
                                               KVM_X86_DISABLE_EXITS_CSTATE)
 
+#define KVM_X86_USERSPACE_EXIT_MWAIT	     (1 << 0)
+#define KVM_X86_USERSPACE_EXIT_HLT	     (1 << 1)
+#define KVM_X86_USERSPACE_VALID_EXITS        (KVM_X86_USERSPACE_EXIT_MWAIT | \
+                                              KVM_X86_USERSPACE_EXIT_HLT)
+
 /* for KVM_ENABLE_CAP */
 struct kvm_enable_cap {
 	/* in */
@@ -1192,6 +1199,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_COUNTER_OFFSET 227
 #define KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE 228
 #define KVM_CAP_ARM_SUPPORTED_BLOCK_SIZES 229
+#define KVM_CAP_X86_USERSPACE_EXITS 230
 
 #ifdef KVM_CAP_IRQ_ROUTING

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