On Wed, Jun 03, 2020 at 11:22:47AM +0100, Daniel P. Berrangé wrote:
> We need to be able to cast from virObjectEventPtr to one of
> its many subclasses. Some of these subclasses have 8 byte
> alignment on 32-bit platforms, but virObjectEventPtr only
> has 4 byte alignment.
>
> Previously the virObject base class had 8 byte alignment
> but this dropped to 4 byte when converted to inherit from
> GObject. This introduces cast alignment warnings on 32-bit:
>
> ../../src/conf/domain_event.c: In function 'virDomainEventDispatchDefaultFunc':
> ../../src/conf/domain_event.c:1656:30: error: cast increases required alignment of target type [-Werror=cast-align]
> 1656 | rtcChangeEvent = (virDomainEventRTCChangePtr)event;
> | ^
> ../../src/conf/domain_event.c:1785:34: error: cast increases required alignment of target type [-Werror=cast-align]
> 1785 | balloonChangeEvent = (virDomainEventBalloonChangePtr)event;
> | ^
> ../../src/conf/domain_event.c:1896:35: error: cast increases required alignment of target type [-Werror=cast-align]
> 1896 | blockThresholdEvent = (virDomainEventBlockThresholdPtr)event;
> | ^
> ../../src/conf/domain_event.c: In function 'virDomainQemuMonitorEventDispatchFunc':
> ../../src/conf/domain_event.c:1974:24: error: cast increases required alignment of target type [-Werror=cast-align]
> 1974 | qemuMonitorEvent = (virDomainQemuMonitorEventPtr)event;
> | ^
> ../../src/conf/domain_event.c: In function 'virDomainQemuMonitorEventFilter':
> ../../src/conf/domain_event.c:2179:20: error: cast increases required alignment of target type [-Werror=cast-align]
> 2179 | monitorEvent = (virDomainQemuMonitorEventPtr) event;
> | ^
>
> Forcing 8-byte alignment on virObjectEventPtr removes the
> alignment increase during casts to subclasses.
>
> Signed-off-by: Daniel P. Berrangé <berrange@xxxxxxxxxx>
> ---
>
> Technically a build-breaker, but since we don't have any existing
> usage of __attribute__((aligned)), I wanted to get a second opinion
> on this approach.
>
> One alternative approach would be to switch one of the current "int"
> fields in virObjectEvent to "long long".
I personally prefer using __attribute__. We already use that GCC
extension so I don't see any reason not using another part of that
extension.
>
> src/conf/object_event_private.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/src/conf/object_event_private.h b/src/conf/object_event_private.h
> index b31441c53a..126464a9a5 100644
> --- a/src/conf/object_event_private.h
> +++ b/src/conf/object_event_private.h
> @@ -42,7 +42,7 @@ typedef void
> virConnectObjectEventGenericCallback cb,
> void *cbopaque);
>
> -struct _virObjectEvent {
> +struct __attribute__((aligned(4))) _virObjectEvent {
> virObject parent;
> int eventID;
> virObjectMeta meta;
Reviewed-by: Pavel Hrdina <phrdina@xxxxxxxxxx>
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