[libvirt] [PATCH] object: require maximal alignment in base class

[Michal Privoznik]

On 13.12.2013 00:07, Eric Blake wrote:
> Recent changes to events (commit 8a29ffcf) resulted in new compile
> failures on some targets (such as ARM OMAP5):
> conf/domain_event.c: In function 'virDomainEventDispatchDefaultFunc':
> conf/domain_event.c:1198:30: error: cast increases required alignment of
> target type [-Werror=cast-align]
> conf/domain_event.c:1314:34: error: cast increases required alignment of
> target type [-Werror=cast-align]
> cc1: all warnings being treated as errors
> 
> The error is due to alignment; the base class is merely aligned
> to the worst of 'int' and 'void*', while the child class must
> be aligned to a 'long long'.  The solution is to include a
> 'long long' (and for good measure, a function pointer) in the
> base class to ensure correct alignment regardless of what a
> child class may add, but to wrap the inclusion in a union so
> as to not incur any wasted space.  On a typical x86_64 platform,
> the base class remains 16 bytes; on i686, the base class remains
> 12 bytes; and on the impacted ARM platform, the base class grows
> from 12 bytes to 16 bytes due to the increase of alignment from
> 4 to 8 bytes.
> 
> Reported by Michele Paolino and others.
> 
> * src/util/virobject.h (_virObject): Use a union to ensure that
> subclasses never have stricter alignment than the parent.
> * src/util/virobject.c (virObjectNew, virObjectUnref)
> (virObjectRef): Adjust clients.
> * src/libvirt.c (virConnectRef, virDomainRef, virNetworkRef)
> (virInterfaceRef, virStoragePoolRef, virStorageVolRef)
> (virNodeDeviceRef, virSecretRef, virStreamRef, virNWFilterRef)
> (virDomainSnapshotRef): Likewise.
> * src/qemu/qemu_monitor.c (qemuMonitorOpenInternal)
> (qemuMonitorClose): Likewise.
> 
> Signed-off-by: Eric Blake <eblake at redhat.com>
> ---
> 
> Even though this fixes a build-breaker, I'd rather that someone
> actually experiencing the build failure test that this fixes
> the problem for them before I push (I don't have easy access to
> hardware exhibiting the problem).
> 
>  src/libvirt.c           | 22 +++++++++++-----------
>  src/qemu/qemu_monitor.c |  4 ++--
>  src/util/virobject.c    | 10 +++++-----
>  src/util/virobject.h    | 16 ++++++++++++++--
>  4 files changed, 32 insertions(+), 20 deletions(-)
> 

> diff --git a/src/util/virobject.h b/src/util/virobject.h
> index 3a08f10..d571b5c 100644
> --- a/src/util/virobject.h
> +++ b/src/util/virobject.h
> @@ -36,9 +36,21 @@ typedef virObjectLockable *virObjectLockablePtr;
> 
>  typedef void (*virObjectDisposeCallback)(void *obj);
> 
> +/* Most code should not play with the contents of this struct; however,
> + * the struct itself is public so that it can be embedded as the first
> + * field of a subclassed object.  */
>  struct _virObject {
> -    unsigned int magic;
> -    int refs;
> +    /* Ensure correct alignment of this and all subclasses, even on
> +     * platforms where 'long long' or function pointers have stricter
> +     * requirements than 'void *'.  */
> +    union {
> +        long long dummy_align1;
> +        void (*dummy_align2) (void);
> +        struct {
> +            unsigned int magic;
> +            int refs;
> +        } s;
> +    } u;
>      virClassPtr klass;
>  };
> 

Beside the comment above union I got the same diff. I did compile check
on my armv6l box and it passed.

ACK

Michal