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Re: [PATCH 2/2] cpu/core: Fix "help" of CPU core device types

From: Thomas Huth
Subject: Re: [PATCH 2/2] cpu/core: Fix "help" of CPU core device types
Date: Sat, 10 Apr 2021 06:53:19 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0

On 09/04/2021 18.03, Greg Kurz wrote:
Calling qdev_get_machine() from a QOM instance_init function is
fragile because we can't be sure the machine object actually
exists. And this happens to break when passing ",help" on the
command line to get the list of properties for a CPU core
device types :

$ ./qemu-system-ppc64 -device power8_v2.0-spapr-cpu-core,help
qemu-system-ppc64: ../../hw/core/machine.c:1290:
  qdev_get_machine: Assertion `machine != NULL' failed.
Aborted (core dumped)

This used to work before QEMU 5.0, but commit 3df261b6676b
unwillingly introduced a subtle regression : the above command
line needs to create an instance but the instance_init function
of the base class calls qdev_get_machine() before
qemu_create_machine() has been called, which is a programming bug.

Use current_machine instead. It is okay to skip the setting of
nr_thread in this case since only its type is displayed.

Reported-by: Thomas Huth <thuth@redhat.com>
Fixes: 3df261b6676b ("softmmu/vl.c: Handle '-cpu help' and '-device help' before 'no 
default machine'")
Cc: peter.maydell@linaro.org
Signed-off-by: Greg Kurz <groug@kaod.org>
  hw/cpu/core.c | 10 ++++++++--
  1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/hw/cpu/core.c b/hw/cpu/core.c
index 92d3b2fbad62..987607515574 100644
--- a/hw/cpu/core.c
+++ b/hw/cpu/core.c
@@ -66,10 +66,16 @@ static void core_prop_set_nr_threads(Object *obj, Visitor 
*v, const char *name,
static void cpu_core_instance_init(Object *obj)
-    MachineState *ms = MACHINE(qdev_get_machine());
      CPUCore *core = CPU_CORE(obj);
- core->nr_threads = ms->smp.threads;
+    /*
+     * Only '-device something-cpu-core,help' can get us there before
+     * the machine has been created. We don't care to set nr_threads
+     * in this case since it isn't used afterwards.
+     */
+    if (current_machine) {
+        core->nr_threads = current_machine->smp.threads;
+    }

Ack for QEMU 6.0 to get rid of the crash. But note that using current_machine might also be wrong in some cases. It's e.g. possible that the user started QEMU with a different machine type (e.g. -M g3beige) and then uses some QOM commands to introspect the available devices - in that case, this instance_init function will be executed with current_machine pointing to a G3 Mac - which is certainly also not what we want here. It likely does not crash, but still ... using current_machine or qdev_get_machine() in an instance_init() function is just wrong. This nr_thread stuff should likely be done in the realize function instead.


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