[Qemu-devel] [PATCH] return default values for apic probe functions.

[Glauber Costa <glommer@redhat.com>]

As KVM cpus runs on threads, it is possible that
we call kvm_load_registers() from a cpu thread, while the
apic has not yet fully initialized. kvm_load_registers() is called
from ap_main_loop.

This is not a problem when we're starting the whole machine together,
but is a problem for hotplug, since we don't have the protection
of the locks that protect machine initialization. Currently, some executions
of cpu hotplug on rainy sundays fail with a segfault.

Moving apic initialization to before kvm_init_vpcu proved fruitful,
as there are some dependencies involved. (kvm irqchip would fail to
initialize).

This patch provides default values to be used for tpr and apic_base,
that will be returned when the apic is not yet properly initialized.
It is aimed at kvm, where the problem exists, but it could equally be
used for qemu too, if there is agreement.

Signed-off-by: Glauber Costa <glommer@redhat.com>
---
 qemu/hw/apic.c |   12 ++++++++++--
 1 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/qemu/hw/apic.c b/qemu/hw/apic.c
index b926508..06fb9b5 100644
--- a/qemu/hw/apic.c
+++ b/qemu/hw/apic.c
@@ -301,7 +301,12 @@ uint64_t cpu_get_apic_base(CPUState *env)
 #ifdef DEBUG_APIC
     printf("cpu_get_apic_base: %016" PRIx64 "\n", (uint64_t)s->apicbase);
 #endif
-    return s->apicbase;
+    if (s) {
+        return s->apicbase;
+    }
+    else {
+        return 0xfee00000 | MSR_IA32_APICBASE_ENABLE;
+    }
 }
 
 void cpu_set_apic_tpr(CPUX86State *env, uint8_t val)
@@ -314,7 +319,10 @@ void cpu_set_apic_tpr(CPUX86State *env, uint8_t val)
 uint8_t cpu_get_apic_tpr(CPUX86State *env)
 {
     APICState *s = env->apic_state;
-    return s->tpr >> 4;
+    if (s)
+        return s->tpr >> 4;
+    else
+        return 0;
 }
 
 /* return -1 if no bit is set */
-- 
1.5.6.6