KVM: x86/xen: Fix kvm_xen_has_interrupt() sleeping in kvm_vcpu_block()

In kvm_vcpu_block, the current task is set to TASK_INTERRUPTIBLE before
making a final check whether the vCPU should be woken from HLT by any
incoming interrupt.

This is a problem for the get_user() in __kvm_xen_has_interrupt(), which
really shouldn't be sleeping when the task state has already been set.
I think it's actually harmless as it would just manifest itself as a
spurious wakeup, but it's causing a debug warning:

[  230.963649] do not call blocking ops when !TASK_RUNNING; state=1 set at [<00000000b6bcdbc9>] prepare_to_swait_exclusive+0x30/0x80

Fix the warning by turning it into an *explicit* spurious wakeup. When
invoked with !task_is_running(current) (and we might as well add
in_atomic() there while we're at it), just return 1 to indicate that
an IRQ is pending, which will cause a wakeup and then something will
call it again in a context that *can* sleep so it can fault the page
back in.

Cc: stable@vger.kernel.org
Fixes: 40da8ccd ("KVM: x86/xen: Add event channel interrupt vector upcall")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>

Message-Id: <168bf8c689561da904e48e2ff5ae4713eaef9e2d.camel@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/xen.c

@@ -190,6 +190,7 @@ void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, int state)
 
 int __kvm_xen_has_interrupt(struct kvm_vcpu *v)
 {
+	int err;
 	u8 rc = 0;
 
 	/*
@@ -216,13 +217,29 @@ int __kvm_xen_has_interrupt(struct kvm_vcpu *v)
 	if (likely(slots->generation == ghc->generation &&
 		   !kvm_is_error_hva(ghc->hva) && ghc->memslot)) {
 		/* Fast path */
-		__get_user(rc, (u8 __user *)ghc->hva + offset);
-	} else {
-		/* Slow path */
-		kvm_read_guest_offset_cached(v->kvm, ghc, &rc, offset,
-					     sizeof(rc));
+		pagefault_disable();
+		err = __get_user(rc, (u8 __user *)ghc->hva + offset);
+		pagefault_enable();
+		if (!err)
+			return rc;
 	}
 
+	/* Slow path */
+
+	/*
+	 * This function gets called from kvm_vcpu_block() after setting the
+	 * task to TASK_INTERRUPTIBLE, to see if it needs to wake immediately
+	 * from a HLT. So we really mustn't sleep. If the page ended up absent
+	 * at that point, just return 1 in order to trigger an immediate wake,
+	 * and we'll end up getting called again from a context where we *can*
+	 * fault in the page and wait for it.
+	 */
+	if (in_atomic() || !task_is_running(current))
+		return 1;
+
+	kvm_read_guest_offset_cached(v->kvm, ghc, &rc, offset,
+				     sizeof(rc));
+
 	return rc;
 }