Re: [Qemu-trivial] [PATCH] srp: Don't use QEMU_PACKED for single elements of a structured type

[Stefan Weil <sw@weilnetz.de>]

Am 15.08.2012 16:16, schrieb Stefan Hajnoczi:
> On Fri, Aug 10, 2012 at 10:03:27PM +0200, Stefan Weil wrote:
>> QEMU_PACKED results in a MinGW compiler warning when it is
>> used for single structure elements:
>>
>> warning: 'gcc_struct' attribute ignored
>>
>> Using QEMU_PACKED for the whole structure avoids the compiler warning
>> without changing the memory layout.
> Quick link for other reviewers:
> http://gcc.gnu.org/onlinedocs/gcc-4.7.1/gcc/Type-Attributes.html#Type-Attributes
>
>> Signed-off-by: Stefan Weil <sw@weilnetz.de>
>> ---
>>   hw/srp.h |    8 ++++----
>>   1 files changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/hw/srp.h b/hw/srp.h
>> index 3009bd5..5e0cad5 100644
>> --- a/hw/srp.h
>> +++ b/hw/srp.h
>> @@ -177,13 +177,13 @@ struct srp_tsk_mgmt {
>>       uint8_t    reserved1[6];
>>       uint64_t   tag;
>>       uint8_t    reserved2[4];
>> -    uint64_t   lun QEMU_PACKED;
>> +    uint64_t   lun;
>>       uint8_t    reserved3[2];
>>       uint8_t    tsk_mgmt_func;
>>       uint8_t    reserved4;
>>       uint64_t   task_tag;
>>       uint8_t    reserved5[8];
>> -};
>> +} QEMU_PACKED;
> Here I actually see a difference for the uint64_t task_tag field.
> Previously it was not packed, now it is packed and because it has 4 *
> uint8_t before it there will be a difference in layout.
>
> Looking at how QEMU accesses srp_tsk_mgmt, I think we're safe because we
> never actually access task_tag?
>
> Ben: Any thoughts on this patch?
>
> Stefan

4 * uint8_t + 4 bytes from the packed lun, so there is no change
for task_tag, it's always on a 8 byte boundary!

Regards,
Stefan