Re: [Qemu-devel] [PATCH v2 1/3] hw/s390x/ipl: Fix alignment problems of S390IPLState members

[Cornelia Huck]

On Wed, 26 Sep 2018 11:46:22 +0200
Thomas Huth <address@hidden> wrote:

> On 2018-09-26 11:42, Cornelia Huck wrote:
> > On Wed, 26 Sep 2018 09:38:45 +0200
> > Thomas Huth <address@hidden> wrote:
> >   
> >> The IplParameterBlock and QemuIplParameters structures are declared
> >> with QEMU_PACKED, so the compiler assumes that the structures do not
> >> need to be aligned in memory. Since the are listed after a "bool"
> >> within the S390IPLState, the IplParameterBlock and QemuIplParameters
> >> are also indeed mis-aligned in memory. This causes problems on Sparc
> >> during migration, since we use VMSTATE_UINT16 in vmstate_iplb to access
> >> the devno member for example, and the corresponding migration functions
> >> (like qemu_get_be16s) then try to access a 16-bit value from a mis-
> >> aligned memory address.
> >> The easiest solution to fix this problem is to move the packed structures
> >> to the beginning of the S390IPLState. Also add some additional comments
> >> here to prevent that this problem will be introduced again in the future.
> >>
> >> Signed-off-by: Thomas Huth <address@hidden>
> >> ---
> >>  hw/s390x/ipl.h | 5 +++--
> >>  1 file changed, 3 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/hw/s390x/ipl.h b/hw/s390x/ipl.h
> >> index 4e87b89..b3a07a1 100644
> >> --- a/hw/s390x/ipl.h
> >> +++ b/hw/s390x/ipl.h
> >> @@ -132,15 +132,15 @@ typedef struct QemuIplParameters QemuIplParameters;
> >>  struct S390IPLState {
> >>      /*< private >*/
> >>      DeviceState parent_obj;
> >> +    IplParameterBlock iplb;
> >> +    QemuIplParameters qipl;  
> > 
> > Hm... this is not quite the beginning of the structure; what am I
> > missing?  
> 
> DeviceState of course has to stay first for QOM reasons. But since it is
> a non-packed struct, we can be sure that it will be padded to the
> correct alignment at the end. If not, the QEMU_BUILD_BUG_MSG in this
> patch will tell us.

What about adding that explanation to the commit message?

[I can do that.]