Re: [RFC PATCH-for-5.2] hw/i386/acpi-build: Fix maybe-uninitialized error when ACPI hotplug off

[Michael S. Tsirkin]

On Sat, Nov 07, 2020 at 11:29:40AM +0100, Philippe Mathieu-Daudé wrote:
> 'method' can be left uninitialized. Restrict some code to be used
> only in the "if (bsel || pcihp_bridge_en)" block statement.
> 
> This fixes (gcc 9.3.0 on Ubuntu):
> 
>   ../hw/i386/acpi-build.c: In function 'build_append_pci_bus_devices':
>   ../hw/i386/acpi-build.c:496:9: error: 'method' may be used uninitialized
>   in this function [-Werror=maybe-uninitialized]
>     496 |         aml_append(parent_scope, method);
>         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   cc1: all warnings being treated as errors

This code is already in the "if (bsel || pcihp_bridge_en)" block statement.
the compiler just isn't smart enough to figure it out.
I do like the change though, I think repeating the condition
is fragile, it is better to have it in a single place.
Pls repost with method can be left -> gcc 9 thinks that method can be left,
no RFC tag, and I will apply.

> Fixes: df4008c9c59 ("piix4: don't reserve hw resources when hotplug is off 
> globally")
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
> RFC because I have no clue about this code
> ---
>  hw/i386/acpi-build.c | 45 +++++++++++++++++++++-----------------------
>  1 file changed, 21 insertions(+), 24 deletions(-)
> 
> diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
> index 4f66642d887..1f5c2112452 100644
> --- a/hw/i386/acpi-build.c
> +++ b/hw/i386/acpi-build.c
> @@ -465,34 +465,31 @@ static void build_append_pci_bus_devices(Aml 
> *parent_scope, PCIBus *bus,
>       */
>      if (bsel || pcihp_bridge_en) {
>          method = aml_method("PCNT", 0, AML_NOTSERIALIZED);
> -    }
> -    /* If bus supports hotplug select it and notify about local events */
> -    if (bsel) {
> -        uint64_t bsel_val = qnum_get_uint(qobject_to(QNum, bsel));
>  
> -        aml_append(method, aml_store(aml_int(bsel_val), aml_name("BNUM")));
> -        aml_append(method,
> -            aml_call2("DVNT", aml_name("PCIU"), aml_int(1) /* Device Check 
> */)
> -        );
> -        aml_append(method,
> -            aml_call2("DVNT", aml_name("PCID"), aml_int(3)/* Eject Request 
> */)
> -        );
> -    }
> +        /* If bus supports hotplug select it and notify about local events */
> +        if (bsel) {
> +            uint64_t bsel_val = qnum_get_uint(qobject_to(QNum, bsel));
>  
> -    /* Notify about child bus events in any case */
> -    if (pcihp_bridge_en) {
> -        QLIST_FOREACH(sec, &bus->child, sibling) {
> -            int32_t devfn = sec->parent_dev->devfn;
> -
> -            if (pci_bus_is_root(sec) || pci_bus_is_express(sec)) {
> -                continue;
> -            }
> -
> -            aml_append(method, aml_name("^S%.02X.PCNT", devfn));
> +            aml_append(method, aml_store(aml_int(bsel_val), 
> aml_name("BNUM")));
> +            aml_append(method, aml_call2("DVNT", aml_name("PCIU"),
> +                                         aml_int(1))); /* Device Check */
> +            aml_append(method, aml_call2("DVNT", aml_name("PCID"),
> +                                         aml_int(3))); /* Eject Request */
> +        }
> +
> +        /* Notify about child bus events in any case */
> +        if (pcihp_bridge_en) {
> +            QLIST_FOREACH(sec, &bus->child, sibling) {
> +                int32_t devfn = sec->parent_dev->devfn;
> +
> +                if (pci_bus_is_root(sec) || pci_bus_is_express(sec)) {
> +                    continue;
> +                }
> +
> +                aml_append(method, aml_name("^S%.02X.PCNT", devfn));
> +            }
>          }
> -    }
>  
> -    if (bsel || pcihp_bridge_en) {
>          aml_append(parent_scope, method);
>      }
>      qobject_unref(bsel);
> -- 
> 2.26.2