xen/vcpu: Handle xen_vcpu_setup() failure in hotplug

The hypercall VCPUOP_register_vcpu_info can fail. This failure is
handled by making per_cpu(xen_vcpu, cpu) point to its shared_info
slot and those without one (cpu >= MAX_VIRT_CPUS) be NULL.

For PVH/PVHVM, this is not enough, because we also need to pull
these VCPUs out of circulation.

Fix for PVH/PVHVM: on registration failure in the cpuhp prepare
callback (xen_cpu_up_prepare_hvm()), return an error to the cpuhp
state-machine so it can fail the CPU init.

Fix for PV: the registration happens before smp_init(), so, in the
failure case we clamp setup_max_cpus and limit the number of VCPUs
that smp_init() will bring-up to MAX_VIRT_CPUS.
This is functionally correct but it makes the code a bit simpler
if we get rid of this explicit clamping: for VCPUs that don't have
valid xen_vcpu, fail the CPU init in the cpuhp prepare callback
(xen_cpu_up_prepare_pv()).

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

arch/x86/xen/enlighten.c

@@ -106,8 +106,10 @@ int xen_cpuhp_setup(int (*cpu_up_prepare_cb)(unsigned int),
 	return rc >= 0 ? 0 : rc;
 }
 
-static void xen_vcpu_setup_restore(int cpu)
+static int xen_vcpu_setup_restore(int cpu)
 {
+	int rc = 0;
+
 	/* Any per_cpu(xen_vcpu) is stale, so reset it */
 	xen_vcpu_info_reset(cpu);
 
@@ -117,8 +119,10 @@ static void xen_vcpu_setup_restore(int cpu)
 	 */
 	if (xen_pv_domain() ||
 	    (xen_hvm_domain() && cpu_online(cpu))) {
-		xen_vcpu_setup(cpu);
+		rc = xen_vcpu_setup(cpu);
 	}
+
+	return rc;
 }
 
 /*
@@ -128,7 +132,7 @@ static void xen_vcpu_setup_restore(int cpu)
  */
 void xen_vcpu_restore(void)
 {
-	int cpu;
+	int cpu, rc;
 
 	for_each_possible_cpu(cpu) {
 		bool other_cpu = (cpu != smp_processor_id());
@@ -148,22 +152,25 @@ void xen_vcpu_restore(void)
 		if (xen_pv_domain() || xen_feature(XENFEAT_hvm_safe_pvclock))
 			xen_setup_runstate_info(cpu);
 
-		xen_vcpu_setup_restore(cpu);
-
-		if (other_cpu && is_up &&
+		rc = xen_vcpu_setup_restore(cpu);
+		if (rc)
+			pr_emerg_once("vcpu restore failed for cpu=%d err=%d. "
+					"System will hang.\n", cpu, rc);
+		/*
+		 * In case xen_vcpu_setup_restore() fails, do not bring up the
+		 * VCPU. This helps us avoid the resulting OOPS when the VCPU
+		 * accesses pvclock_vcpu_time via xen_vcpu (which is NULL.)
+		 * Note that this does not improve the situation much -- now the
+		 * VM hangs instead of OOPSing -- with the VCPUs that did not
+		 * fail, spinning in stop_machine(), waiting for the failed
+		 * VCPUs to come up.
+		 */
+		if (other_cpu && is_up && (rc == 0) &&
 		    HYPERVISOR_vcpu_op(VCPUOP_up, xen_vcpu_nr(cpu), NULL))
 			BUG();
 	}
 }
 
-static void clamp_max_cpus(void)
-{
-#ifdef CONFIG_SMP
-	if (setup_max_cpus > MAX_VIRT_CPUS)
-		setup_max_cpus = MAX_VIRT_CPUS;
-#endif
-}
-
 void xen_vcpu_info_reset(int cpu)
 {
 	if (xen_vcpu_nr(cpu) < MAX_VIRT_CPUS) {
@@ -175,7 +182,7 @@ void xen_vcpu_info_reset(int cpu)
 	}
 }
 
-void xen_vcpu_setup(int cpu)
+int xen_vcpu_setup(int cpu)
 {
 	struct vcpu_register_vcpu_info info;
 	int err;
@@ -196,7 +203,7 @@ void xen_vcpu_setup(int cpu)
 	 */
 	if (xen_hvm_domain()) {
 		if (per_cpu(xen_vcpu, cpu) == &per_cpu(xen_vcpu_info, cpu))
-			return;
+			return 0;
 	}
 
 	if (xen_have_vcpu_info_placement) {
@@ -230,11 +237,10 @@ void xen_vcpu_setup(int cpu)
 		}
 	}
 
-	if (!xen_have_vcpu_info_placement) {
-		if (cpu >= MAX_VIRT_CPUS)
-			clamp_max_cpus();
+	if (!xen_have_vcpu_info_placement)
 		xen_vcpu_info_reset(cpu);
-	}
+
+	return ((per_cpu(xen_vcpu, cpu) == NULL) ? -ENODEV : 0);
 }
 
 void xen_reboot(int reason)

arch/x86/xen/enlighten_hvm.c

@@ -89,7 +89,7 @@ static void xen_hvm_crash_shutdown(struct pt_regs *regs)
 
 static int xen_cpu_up_prepare_hvm(unsigned int cpu)
 {
-	int rc;
+	int rc = 0;
 
 	/*
 	 * This can happen if CPU was offlined earlier and
@@ -104,7 +104,9 @@ static int xen_cpu_up_prepare_hvm(unsigned int cpu)
 		per_cpu(xen_vcpu_id, cpu) = cpu_acpi_id(cpu);
 	else
 		per_cpu(xen_vcpu_id, cpu) = cpu;
-	xen_vcpu_setup(cpu);
+	rc = xen_vcpu_setup(cpu);
+	if (rc)
+		return rc;
 
 	if (xen_have_vector_callback && xen_feature(XENFEAT_hvm_safe_pvclock))
 		xen_setup_timer(cpu);
@@ -113,9 +115,8 @@ static int xen_cpu_up_prepare_hvm(unsigned int cpu)
 	if (rc) {
 		WARN(1, "xen_smp_intr_init() for CPU %d failed: %d\n",
 		     cpu, rc);
-		return rc;
 	}
-	return 0;
+	return rc;
 }
 
 static int xen_cpu_dead_hvm(unsigned int cpu)

arch/x86/xen/enlighten_pv.c

@@ -958,7 +958,16 @@ void __ref xen_setup_vcpu_info_placement(void)
 	for_each_possible_cpu(cpu) {
 		/* Set up direct vCPU id mapping for PV guests. */
 		per_cpu(xen_vcpu_id, cpu) = cpu;
-		xen_vcpu_setup(cpu);
+
+		/*
+		 * xen_vcpu_setup(cpu) can fail  -- in which case it
+		 * falls back to the shared_info version for cpus
+		 * where xen_vcpu_nr(cpu) < MAX_VIRT_CPUS.
+		 *
+		 * xen_cpu_up_prepare_pv() handles the rest by failing
+		 * them in hotplug.
+		 */
+		(void) xen_vcpu_setup(cpu);
 	}
 
 	/*
@@ -1432,6 +1441,9 @@ static int xen_cpu_up_prepare_pv(unsigned int cpu)
 {
 	int rc;
 
+	if (per_cpu(xen_vcpu, cpu) == NULL)
+		return -ENODEV;
+
 	xen_setup_timer(cpu);
 
 	rc = xen_smp_intr_init(cpu);

arch/x86/xen/xen-ops.h

@@ -78,7 +78,7 @@ bool xen_vcpu_stolen(int vcpu);
 
 extern int xen_have_vcpu_info_placement;
 
-void xen_vcpu_setup(int cpu);
+int xen_vcpu_setup(int cpu);
 void xen_vcpu_info_reset(int cpu);
 void xen_setup_vcpu_info_placement(void);