Re: [Qemu-block] [Qemu-devel] [PATCH v3 11/12] pc: Support firmware configuration with -blockdev

[Markus Armbruster]

Paolo Bonzini <address@hidden> writes:

> On 08/03/19 14:14, Markus Armbruster wrote:
>> The PC machines put firmware in ROM by default.  To get it put into
>> flash memory (required by OVMF), you have to use -drive
>> if=pflash,unit=0,... and optionally -drive if=pflash,unit=1,...
>> 
>> Why two -drive?  This permits setting up one part of the flash memory
>> read-only, and the other part read/write.  Below the hood, it creates
>> two separate flash devices, because we were too lazy to improve our
>> flash device models to support sector protection.
>
> Not just that, it avoids the need to "upgrade the firmware" of old
> virtual machines.  Instead, you just need to upgrade the files on the
> host and restart QEMU, just like with non-UEFI firmware.

True.  I'll insert "It also makes upgrading firmware on the host
easier."

>> This is actually an instance of a wider problem: our general device
>> configuration interface doesn't cover onboard devices.  Instead, we
>> have a zoo of ad hoc interfaces that are much more limited.  Some of
>> them we'd rather deprecate (-drive, -net), but can't until we have
>> suitable replacements.
>
> -net already has a better replacement, -nic, which is basically "like
> -netdev but also tell board creation code about it".  I suppose you'd
> prefer that it were also accessible as "-netdev ...,id=foo -machine
> net0=foo"?

I got confused.  Or rather, my mind was stuck in the past.  Along with
parts of our UI, to be frank.  Yes, -net already has suitable
replacements (-nic and -netdev hubport).  We've kept around -net anyway.
If this situation confuses me, chances are it confuses users, too.  I
think we should deprecate it.

I'll drop -net from my parenthesis.

Restoring a bit more quoted material for later:

   +void pc_system_flash_create(PCMachineState *pcms)
   +{
   +    PCMachineClass *pcmc = PC_MACHINE_GET_CLASS(pcms);
   +
   +    if (pcmc->pci_enabled) {
   +        pcms->flash[0] = pc_pflash_create("system.flash0");
   +        pcms->flash[1] = pc_pflash_create("system.flash1");
   +        object_property_add_alias(OBJECT(pcms), "pflash0",
   +                                  OBJECT(pcms->flash[0]), "drive",
   +                                  &error_abort);
   +        object_property_add_alias(OBJECT(pcms), "pflash1",
   +                                  OBJECT(pcms->flash[1]), "drive",
   +                                  &error_abort);
   +    }
   +}
   +
>> +static void pc_system_flash_cleanup_unused(PCMachineState *pcms)
>> +{
>> +    char *prop_name;
>> +    int i;
>> +    Object *dev_obj;
>> +
>> +    assert(PC_MACHINE_GET_CLASS(pcms)->pci_enabled);
>> +
>> +    for (i = 0; i < ARRAY_SIZE(pcms->flash); i++) {
>> +        dev_obj = OBJECT(pcms->flash[i]);
>> +        if (!object_property_get_bool(dev_obj, "realized", &error_abort)) {
>> +            prop_name = g_strdup_printf("pflash%d", i);
>> +            object_property_del(OBJECT(pcms), prop_name, &error_abort);
>
> Why didn't this already call object->class->unparent?

Let's see...

    void object_property_del(Object *obj, const char *name, Error **errp)
    {
        ObjectProperty *prop = g_hash_table_lookup(obj->properties, name);

        if (!prop) {
            error_setg(errp, "Property '.%s' not found", name);
            return;
        }

        if (prop->release) {
            prop->release(obj, name, prop->opaque);
        }
        g_hash_table_remove(obj->properties, name);
    }

The only thing that could possibly call object->class->unparent() is
prop->release().  In gdb:

    (gdb) p *prop
    $1 = {name = 0x555556a4aa20 "pflash0", type = 0x555556a4aa40 "str", 
      description = 0x555556a4aa80 "Node name or ID of a block device to use as 
a backend", get = 0x555555d02734 <property_get_alias>, 
      set = 0x555555d0277a <property_set_alias>, 
      resolve = 0x555555d027c0 <property_resolve_alias>, 
      release = 0x555555d027f8 <property_release_alias>, opaque = 
0x555556a4a990}

property_release_alias() doesn't:

    static void property_release_alias(Object *obj, const char *name, void 
*opaque)
    {
        AliasProperty *prop = opaque;

        g_free(prop->target_name);
        g_free(prop);
    }

Remember, the machine property is merely an alias that forwards to the
pflash device's property.

>> +            g_free(prop_name);
>> +            /*
>> +             * Delete @dev_obj.  Straight object_unref() is wrong,
>> +             * since it leaves dangling references in the parent bus
>> +             * behind.  object_unparent() doesn't work, either: since
>> +             * @dev_obj hasn't been realized, dev_obj->parent is null,
>> +             * and object_unparent() does nothing.
>
> Does it work if you add the device yourself as a child of /machine,
> instead of relying on /machine/unattached?

I figure you're suggesting something like this incremental patch:

   diff --git a/hw/i386/pc_sysfw.c b/hw/i386/pc_sysfw.c
   index ccf2221acb..9d3a80c51a 100644
   --- a/hw/i386/pc_sysfw.c
   +++ b/hw/i386/pc_sysfw.c
   @@ -99,6 +99,10 @@ void pc_system_flash_create(PCMachineState *pcms)
        if (pcmc->pci_enabled) {
            pcms->flash[0] = pc_pflash_create("system.flash0");
            pcms->flash[1] = pc_pflash_create("system.flash1");
   +        object_property_add_child(OBJECT(pcms), "pfl0",
   +                                  OBJECT(pcms->flash[0]), &error_abort);
   +        object_property_add_child(OBJECT(pcms), "pfl1",
   +                                  OBJECT(pcms->flash[1]), &error_abort);
            object_property_add_alias(OBJECT(pcms), "pflash0",
                                      OBJECT(pcms->flash[0]), "drive",
                                      &error_abort);
   @@ -122,19 +126,7 @@ static void 
pc_system_flash_cleanup_unused(PCMachineState *pcms)
                prop_name = g_strdup_printf("pflash%d", i);
                object_property_del(OBJECT(pcms), prop_name, &error_abort);
                g_free(prop_name);
   -            /*
   -             * Delete @dev_obj.  Straight object_unref() is wrong,
   -             * since it leaves dangling references in the parent bus
   -             * behind.  object_unparent() doesn't work, either: since
   -             * @dev_obj hasn't been realized, dev_obj->parent is null,
   -             * and object_unparent() does nothing.  DeviceClass method
   -             * device_unparent() works, but we have to take a
   -             * temporary reference, or else device_unparent() commits
   -             * a use-after-free error.
   -             */
   -            object_ref(dev_obj);
   -            object_get_class(dev_obj)->unparent(dev_obj);
   -            object_unref(dev_obj);
   +            object_unparent(dev_obj);
                pcms->flash[i] = NULL;
            }
        }

Passes "make check" and "info qtree" looks good both with and without
-machine pflash0,...

I'm not exactly happy with "pfl0" and "pfl1".  Got a favorite color for
my bikeshed?

>>   DeviceClass method
>> +             * device_unparent() works, but we have to take a
>> +             * temporary reference, or else device_unparent() commits
>> +             * a use-after-free error.
>> +             */
>> +            object_ref(dev_obj);
>> +            object_get_class(dev_obj)->unparent(dev_obj);
>> +            object_unref(dev_obj);
>> +            pcms->flash[i] = NULL;
>> +        }
>
> Paolo

Thank you!

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