[PATCH 2/5] KVM: add KVM_REQ_EXIT request for userspace exit

["Radim Krčmář" <rkrcmar@redhat.com>]

When userspace wants KVM to exit to userspace, it sends a signal.
This has a disadvantage of requiring a change to the signal mask because
the signal needs to be blocked in userspace to stay pending when sending
to self.

Using a request flag allows us to shave 200-300 cycles from every
userspace exit and the speedup grows with NUMA because unblocking
touches shared spinlock.

The disadvantage is that it adds an overhead of one bit check for all
kernel exits.  A quick tracing shows that the ratio of userspace exits
after boot is about 1/5 and in subsequent run of nmap and kernel compile
has about 1/60, so the check should not regress global performance.

All signal_pending() calls are userspace exit requests, so we add a
check for KVM_REQ_EXIT there.  There is one omitted call in kvm_vcpu_run
because KVM_REQ_EXIT is implied in earlier check for requests.

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
---
 arch/x86/kvm/vmx.c       | 2 +-
 arch/x86/kvm/x86.c       | 6 ++++++
 include/linux/kvm_host.h | 8 +++++++-
 include/uapi/linux/kvm.h | 1 +
 virt/kvm/kvm_main.c      | 2 +-
 5 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 17514fe7d2cb..496f6ba1c812 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -5903,7 +5903,7 @@ static int handle_invalid_guest_state(struct kvm_vcpu *vcpu)
 			goto out;
 		}
 
-		if (signal_pending(current))
+		if (kvm_need_exit(vcpu))
 			goto out;
 		if (need_resched())
 			schedule();
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ec7bf2b16948..c5d790fdfc2e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6547,6 +6547,11 @@ static int vcpu_run(struct kvm_vcpu *vcpu)
 			++vcpu->stat.signal_exits;
 			break;
 		}
+		if (unlikely(kvm_has_request(KVM_REQ_EXIT, vcpu))) {
+			r = 0;
+			vcpu->run->exit_reason = KVM_EXIT_REQUEST;
+			break;
+		}
 		if (need_resched()) {
 			srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
 			cond_resched();
@@ -6683,6 +6688,7 @@ out:
 	post_kvm_run_save(vcpu);
 	if (vcpu->sigset_active)
 		sigprocmask(SIG_SETMASK, &sigsaved, NULL);
+	clear_bit(KVM_REQ_EXIT, &vcpu->requests);
 
 	return r;
 }
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 52e388367a26..dcc57171e3ec 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -121,7 +121,7 @@ static inline bool is_error_page(struct page *page)
 #define KVM_REQ_UNHALT             6
 #define KVM_REQ_MMU_SYNC           7
 #define KVM_REQ_CLOCK_UPDATE       8
-#define KVM_REQ_KICK               9
+#define KVM_REQ_EXIT               9
 #define KVM_REQ_DEACTIVATE_FPU    10
 #define KVM_REQ_EVENT             11
 #define KVM_REQ_APF_HALT          12
@@ -1104,6 +1104,12 @@ static inline bool kvm_check_request(int req, struct kvm_vcpu *vcpu)
 	}
 }
 
+static inline bool kvm_need_exit(struct kvm_vcpu *vcpu)
+{
+	return signal_pending(current) ||
+	       kvm_has_request(KVM_REQ_EXIT, vcpu);
+}
+
 extern bool kvm_rebooting;
 
 struct kvm_device {
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 26daafbba9ec..d996a7cdb4d2 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -184,6 +184,7 @@ struct kvm_s390_skeys {
 #define KVM_EXIT_SYSTEM_EVENT     24
 #define KVM_EXIT_S390_STSI        25
 #define KVM_EXIT_IOAPIC_EOI       26
+#define KVM_EXIT_REQUEST          27
 
 /* For KVM_EXIT_INTERNAL_ERROR */
 /* Emulate instruction failed. */
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 093b3d10b411..b34a328fdac1 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1914,7 +1914,7 @@ static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
 	}
 	if (kvm_cpu_has_pending_timer(vcpu))
 		return -EINTR;
-	if (signal_pending(current))
+	if (kvm_need_exit(vcpu))
 		return -EINTR;
 
 	return 0;
-- 
2.5.0