[PATCH] i386/apic: Add hint on boot failure because of disabling x2APIC

[Zhao Liu <zhao1.liu@intel.com>]

Currently, the Q35 supports up to 4096 vCPUs (since v9.0), but for TCG
cases, if x2APIC is not actively enabled to boot more than 255 vCPUs (
e.g., qemu-system-i386 -M pc-q35-9.0 -smp 666), the following error is
reported:

Unexpected error in apic_common_set_id() at ../hw/intc/apic_common.c:449:
qemu-system-i386: APIC ID 255 requires x2APIC feature in CPU
Aborted (core dumped)

This error can be resolved by setting x2apic=on in -cpu. In order to
better help users deal with this scenario, add the error hint to
instruct users on how to enable the x2apic feature. Then, the error
report becomes the following:

Unexpected error in apic_common_set_id() at ../hw/intc/apic_common.c:448:
qemu-system-i386: APIC ID 255 requires x2APIC feature in CPU
Try x2apic=on in -cpu.
Aborted (core dumped)

Note since @errp is &error_abort, error_append_hint() can't be applied
on @errp. And in order to separate the exact error message from the
(perhaps effectively) hint, adding a hint via error_append_hint() is
also necessary. Therefore, introduce @local_error in
apic_common_set_id() to handle both the error message and the error
hint.

Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
---
 hw/intc/apic_common.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/hw/intc/apic_common.c b/hw/intc/apic_common.c
index d8fc1e2815fe..c13cdd79943d 100644
--- a/hw/intc/apic_common.c
+++ b/hw/intc/apic_common.c
@@ -433,6 +433,7 @@ static void apic_common_set_id(Object *obj, Visitor *v, const char *name,
     APICCommonState *s = APIC_COMMON(obj);
     DeviceState *dev = DEVICE(obj);
     uint32_t value;
+    Error *local_err = NULL;
 
     if (dev->realized) {
         qdev_prop_set_after_realize(dev, name, errp);
@@ -444,7 +445,11 @@ static void apic_common_set_id(Object *obj, Visitor *v, const char *name,
     }
 
     if (value >= 255 && !cpu_has_x2apic_feature(&s->cpu->env)) {
-        error_setg(errp, "APIC ID %d requires x2APIC feature in CPU", value);
+        error_setg(&local_err,
+                   "APIC ID %d requires x2APIC feature in CPU",
+                   value);
+        error_append_hint(&local_err, "Try x2apic=on in -cpu.\n");
+        error_propagate(errp, local_err);
         return;
     }
 
-- 
2.34.1