KVM: Don't block+unblock when halt-polling is successful

Invoke the arch hooks for block+unblock if and only if KVM actually
attempts to block the vCPU.  The only non-nop implementation is on x86,
specifically SVM's AVIC, and there is no need to put the AVIC prior to
halt-polling; KVM x86's kvm_vcpu_has_events() will scour the full vIRR
to find pending IRQs regardless of whether the AVIC is loaded/"running".

The primary motivation is to allow future cleanup to split out "block"
from "halt", but this is also likely a small performance boost on x86 SVM
when halt-polling is successful.

Adjust the post-block path to update "cur" after unblocking, i.e. include
AVIC load time in halt_wait_ns and halt_wait_hist, so that the behavior
is consistent.  Moving just the pre-block arch hook would result in only
the AVIC put latency being included in the halt_wait stats.  There is no
obvious evidence that one way or the other is correct, so just ensure KVM
is consistent.

Note, x86 has two separate paths for handling APICv with respect to vCPU
blocking.  VMX uses hooks in x86's vcpu_block(), while SVM uses the arch
hooks in kvm_vcpu_block().  Prior to this path, the two paths were more
or less functionally identical.  That is very much not the case after
this patch, as the hooks used by VMX _must_ fire before halt-polling.
x86's entire mess will be cleaned up in future patches.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211009021236.4122790-12-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -3306,8 +3306,6 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
 	bool waited = false;
 	u64 block_ns;
 
-	kvm_arch_vcpu_blocking(vcpu);
-
 	start = cur = poll_end = ktime_get();
 	if (do_halt_poll) {
 		ktime_t stop = ktime_add_ns(ktime_get(), vcpu->halt_poll_ns);
@@ -3324,6 +3322,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
 		} while (kvm_vcpu_can_poll(cur, stop));
 	}
 
+	kvm_arch_vcpu_blocking(vcpu);
 
 	prepare_to_rcuwait(wait);
 	for (;;) {
@@ -3336,6 +3335,9 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
 		schedule();
 	}
 	finish_rcuwait(wait);
+
+	kvm_arch_vcpu_unblocking(vcpu);
+
 	cur = ktime_get();
 	if (waited) {
 		vcpu->stat.generic.halt_wait_ns +=
@@ -3344,7 +3346,6 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
 				ktime_to_ns(cur) - ktime_to_ns(poll_end));
 	}
 out:
-	kvm_arch_vcpu_unblocking(vcpu);
 	block_ns = ktime_to_ns(cur) - ktime_to_ns(start);
 
 	/*