KVM: PPC: Book3S HV: Fix race in starting secondary threads

The current dynamic micro-threading code has a race due to which a
secondary thread naps when it is supposed to be running a vcpu. As a
side effect of this, on a guest exit, the primary thread in
kvmppc_wait_for_nap() finds that this secondary thread hasn't cleared
its vcore pointer. This results in "CPU X seems to be stuck!"
warnings.

The race is possible since the primary thread on exiting the guests
only waits for all the secondaries to clear its vcore pointer. It
subsequently expects the secondary threads to enter nap while it
unsplits the core. A secondary thread which hasn't yet entered the nap
will loop in kvm_no_guest until its vcore pointer and the do_nap flag
are unset. Once the core has been unsplit, a new vcpu thread can grab
the core and set the do_nap flag *before* setting the vcore pointers
of the secondary. As a result, the secondary thread will now enter nap
via kvm_unsplit_nap instead of running the guest vcpu.

Fix this by setting the do_nap flag after setting the vcore pointer in
the PACA of the secondary in kvmppc_run_core. Also, ensure that a
secondary thread doesn't nap in kvm_unsplit_nap when the vcore pointer
in its PACA struct is set.

Fixes: b4deba5c41e9
Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kvm/book3s_hv.c

@@ -2411,7 +2411,6 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc)
 				break;
 			cpu_relax();
 		}
-		split_info.do_nap = 1;	/* ask secondaries to nap when done */
 	}
 
 	/* Start all the threads */
@@ -2440,6 +2439,15 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc)
 			thr += pvc->num_threads;
 		}
 	}
+
+	/*
+	 * Ensure that split_info.do_nap is set after setting
+	 * the vcore pointer in the PACA of the secondaries.
+	 */
+	smp_mb();
+	if (cmd_bit)
+		split_info.do_nap = 1;	/* ask secondaries to nap when done */
+
 	/*
 	 * When doing micro-threading, poke the inactive threads as well.
 	 * This gets them to the nap instruction after kvm_do_nap,

arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -421,6 +421,14 @@ kvm_no_guest:
  * whole-core mode, so we need to nap.
  */
 kvm_unsplit_nap:
+	/*
+	 * Ensure that secondary doesn't nap when it has
+	 * its vcore pointer set.
+	 */
+	sync		/* matches smp_mb() before setting split_info.do_nap */
+	ld	r0, HSTATE_KVM_VCORE(r13)
+	cmpdi	r0, 0
+	bne	kvm_no_guest
 	/* clear any pending message */
 BEGIN_FTR_SECTION
 	lis	r6, (PPC_DBELL_SERVER << (63-36))@h