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@redhat.com>
---
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".
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;
--
2.24.1
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@redhat.com> > --- > > 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@redhat.com>
On Wed, Jun 3, 2020 at 12:29 PM Daniel P. Berrangé <berrange@redhat.com> 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@redhat.com> > --- > > 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". > > 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; > -- > 2.24.1 > This is the same error I'm seeing with my patches in CI when converting virStorageSource to GObject except no alignment value seems to fix it. -- Rafael Fonseca
On 6/3/20 12:22 PM, 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@redhat.com> > --- > > 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". > > 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: Michal Privoznik <mprivozn@redhat.com> Michal
On 6/3/20 5:22 AM, Daniel P. Berrangé wrote: > Forcing 8-byte alignment on virObjectEventPtr removes the > alignment increase during casts to subclasses. > > Signed-off-by: Daniel P. Berrangé <berrange@redhat.com> > --- > > 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". > > -struct _virObjectEvent { > +struct __attribute__((aligned(4))) _virObjectEvent { > virObject parent; > int eventID; > virObjectMeta meta; As in this, although it makes the struct larger on 32-bit platforms (which may in turn affect cache usage): struct _virObjectEvent { virObject parent; long long eventID; ... Another possibility: use the extension of unnamed unions (but if we're going to rely on gcc extensions, __attribute__ is nicer than unnamed unions): struct _virObjectEvent { union { virObject parent; long long alignment_; }; int eventID; ... We already limit ourselves to gcc and clang because of __attribute__((cleanup)), so I don't see any problem with your approach. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
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