KVM: race-free exit from KVM_RUN without POSIX signals

The purpose of the KVM_SET_SIGNAL_MASK API is to let userspace "kick"
a VCPU out of KVM_RUN through a POSIX signal.  A signal is attached
to a dummy signal handler; by blocking the signal outside KVM_RUN and
unblocking it inside, this possible race is closed:

          VCPU thread                     service thread
   --------------------------------------------------------------
        check flag
                                          set flag
                                          raise signal
        (signal handler does nothing)
        KVM_RUN

However, one issue with KVM_SET_SIGNAL_MASK is that it has to take
tsk->sighand->siglock on every KVM_RUN.  This lock is often on a
remote NUMA node, because it is on the node of a thread's creator.
Taking this lock can be very expensive if there are many userspace
exits (as is the case for SMP Windows VMs without Hyper-V reference
time counter).

As an alternative, we can put the flag directly in kvm_run so that
KVM can see it:

          VCPU thread                     service thread
   --------------------------------------------------------------
                                          raise signal
        signal handler
          set run->immediate_exit
        KVM_RUN
          check run->immediate_exit

Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virtual/kvm/api.txt

@@ -3389,7 +3389,18 @@ struct kvm_run {
 Request that KVM_RUN return when it becomes possible to inject external
 interrupts into the guest.  Useful in conjunction with KVM_INTERRUPT.
 
-	__u8 padding1[7];
+	__u8 immediate_exit;
+
+This field is polled once when KVM_RUN starts; if non-zero, KVM_RUN
+exits immediately, returning -EINTR.  In the common scenario where a
+signal is used to "kick" a VCPU out of KVM_RUN, this field can be used
+to avoid usage of KVM_SET_SIGNAL_MASK, which has worse scalability.
+Rather than blocking the signal outside KVM_RUN, userspace can set up
+a signal handler that sets run->immediate_exit to a non-zero value.
+
+This field is ignored if KVM_CAP_IMMEDIATE_EXIT is not available.
+
+	__u8 padding1[6];
 
 	/* out */
 	__u32 exit_reason;

arch/arm/kvm/arm.c

@@ -206,6 +206,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_ARM_PSCI_0_2:
 	case KVM_CAP_READONLY_MEM:
 	case KVM_CAP_MP_STATE:
+	case KVM_CAP_IMMEDIATE_EXIT:
 		r = 1;
 		break;
 	case KVM_CAP_COALESCED_MMIO:
@@ -604,6 +605,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
 			return ret;
 	}
 
+	if (run->immediate_exit)
+		return -EINTR;
+
 	if (vcpu->sigset_active)
 		sigprocmask(SIG_SETMASK, &vcpu->sigset, &sigsaved);
 

arch/mips/kvm/mips.c

@@ -397,7 +397,7 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
 
 int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
 {
-	int r = 0;
+	int r = -EINTR;
 	sigset_t sigsaved;
 
 	if (vcpu->sigset_active)
@@ -409,6 +409,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
 		vcpu->mmio_needed = 0;
 	}
 
+	if (run->immediate_exit)
+		goto out;
+
 	lose_fpu(1);
 
 	local_irq_disable();
@@ -429,6 +432,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
 	guest_exit_irqoff();
 	local_irq_enable();
 
+out:
 	if (vcpu->sigset_active)
 		sigprocmask(SIG_SETMASK, &sigsaved, NULL);
 
@@ -1021,6 +1025,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_ENABLE_CAP:
 	case KVM_CAP_READONLY_MEM:
 	case KVM_CAP_SYNC_MMU:
+	case KVM_CAP_IMMEDIATE_EXIT:
 		r = 1;
 		break;
 	case KVM_CAP_COALESCED_MMIO:

arch/powerpc/kvm/powerpc.c

@@ -511,6 +511,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_ONE_REG:
 	case KVM_CAP_IOEVENTFD:
 	case KVM_CAP_DEVICE_CTRL:
+	case KVM_CAP_IMMEDIATE_EXIT:
 		r = 1;
 		break;
 	case KVM_CAP_PPC_PAIRED_SINGLES:
@@ -1117,7 +1118,10 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
 #endif
 	}
 
-	r = kvmppc_vcpu_run(run, vcpu);
+	if (run->immediate_exit)
+		r = -EINTR;
+	else
+		r = kvmppc_vcpu_run(run, vcpu);
 
 	if (vcpu->sigset_active)
 		sigprocmask(SIG_SETMASK, &sigsaved, NULL);

arch/s390/kvm/kvm-s390.c

@@ -370,6 +370,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_S390_IRQCHIP:
 	case KVM_CAP_VM_ATTRIBUTES:
 	case KVM_CAP_MP_STATE:
+	case KVM_CAP_IMMEDIATE_EXIT:
 	case KVM_CAP_S390_INJECT_IRQ:
 	case KVM_CAP_S390_USER_SIGP:
 	case KVM_CAP_S390_USER_STSI:
@@ -2798,6 +2799,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 	int rc;
 	sigset_t sigsaved;
 
+	if (kvm_run->immediate_exit)
+		return -EINTR;
+
 	if (guestdbg_exit_pending(vcpu)) {
 		kvm_s390_prepare_debug_exit(vcpu);
 		return 0;

arch/x86/kvm/x86.c

@@ -2672,6 +2672,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_DISABLE_QUIRKS:
 	case KVM_CAP_SET_BOOT_CPU_ID:
  	case KVM_CAP_SPLIT_IRQCHIP:
+	case KVM_CAP_IMMEDIATE_EXIT:
 #ifdef CONFIG_KVM_DEVICE_ASSIGNMENT
 	case KVM_CAP_ASSIGN_DEV_IRQ:
 	case KVM_CAP_PCI_2_3:
@@ -7202,7 +7203,10 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 	} else
 		WARN_ON(vcpu->arch.pio.count || vcpu->mmio_needed);
 
-	r = vcpu_run(vcpu);
+	if (kvm_run->immediate_exit)
+		r = -EINTR;
+	else
+		r = vcpu_run(vcpu);
 
 out:
 	post_kvm_run_save(vcpu);

include/uapi/linux/kvm.h

@@ -218,7 +218,8 @@ struct kvm_hyperv_exit {
 struct kvm_run {
 	/* in */
 	__u8 request_interrupt_window;
-	__u8 padding1[7];
+	__u8 immediate_exit;
+	__u8 padding1[6];
 
 	/* out */
 	__u32 exit_reason;
@@ -881,6 +882,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_SPAPR_RESIZE_HPT 133
 #define KVM_CAP_PPC_MMU_RADIX 134
 #define KVM_CAP_PPC_MMU_HASH_V3 135
+#define KVM_CAP_IMMEDIATE_EXIT 136
 
 #ifdef KVM_CAP_IRQ_ROUTING