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

[Thomas Huth]

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.

 Thomas