KVM: x86: WARN_ONCE if sending a PV IPI returns a fatal error

KVM hypercalls return a negative value error code in case of a fatal
error, e.g. when the hypercall isn't supported or was made with invalid
parameters.  WARN_ONCE on fatal errors when sending PV IPIs as any such
error all but guarantees an SMP system will hang due to a missing IPI.

Fixes: aaffcfd1 ("KVM: X86: Implement PV IPIs in linux guest")
Cc: stable@vger.kernel.org
Cc: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kernel/kvm.c

@@ -457,6 +457,7 @@ static void __send_ipi_mask(const struct cpumask *mask, int vector)
 #else
 	u64 ipi_bitmap = 0;
 #endif
+	long ret;
 
 	if (cpumask_empty(mask))
 		return;
@@ -482,8 +483,9 @@ static void __send_ipi_mask(const struct cpumask *mask, int vector)
 		} else if (apic_id < min + KVM_IPI_CLUSTER_SIZE) {
 			max = apic_id < max ? max : apic_id;
 		} else {
-			kvm_hypercall4(KVM_HC_SEND_IPI, (unsigned long)ipi_bitmap,
+			ret = kvm_hypercall4(KVM_HC_SEND_IPI, (unsigned long)ipi_bitmap,
 				(unsigned long)(ipi_bitmap >> BITS_PER_LONG), min, icr);
+			WARN_ONCE(ret < 0, "KVM: failed to send PV IPI: %ld", ret);
 			min = max = apic_id;
 			ipi_bitmap = 0;
 		}
@@ -491,8 +493,9 @@ static void __send_ipi_mask(const struct cpumask *mask, int vector)
 	}
 
 	if (ipi_bitmap) {
-		kvm_hypercall4(KVM_HC_SEND_IPI, (unsigned long)ipi_bitmap,
+		ret = kvm_hypercall4(KVM_HC_SEND_IPI, (unsigned long)ipi_bitmap,
 			(unsigned long)(ipi_bitmap >> BITS_PER_LONG), min, icr);
+		WARN_ONCE(ret < 0, "KVM: failed to send PV IPI: %ld", ret);
 	}
 
 	local_irq_restore(flags);