Re: [Qemu-devel] [PATCH 0/7] Fix packing for MinGW with -mms-bitfields

[Stefan Weil <weil@mail.berlios.de>]

Am 29.08.2011 10:34, schrieb TeLeMan:
> On Mon, Aug 29, 2011 at 13:01, Stefan Weil <weil@mail.berlios.de> wrote:
>> Am 28.08.2011 23:43, schrieb Blue Swirl:
>>>
>>> On Sun, Aug 28, 2011 at 8:43 PM, Stefan Weil <weil@mail.berlios.de> 
>>> wrote:
>>>>
>>>> These patches fix the packing of structures which were affected by
>>>> the new compiler attribute -mms-bitfields (which is needed for 
>>>> glib-2.0).
>>>>
>>>> I compiled qemu.exe with and without -mms-bitfields and compared
>>>> the resulting struct alignment using pahole and codiff.
>>>
>>> If a structure is only used internally by QEMU (not used in network,
>>> disk or guest interfaces), changes in padding don't matter. In fact,
>>> in those cases it may be better to remove the packing, because then
>>> the fields may be naturally aligned and that gives better performance
>>> on most architectures. Could you please check if this is the case for
>>> any of the structs?
>>
>> I did this already, but also forward your question to the maintainers.
>> Here is my result:
>>
>> [PATCH 2/7] block/vvfat: Fix packing for w32: needs packing (disk)
>> [PATCH 3/7] acpi: Fix packing for w32: needs packing (bios interface)
>> [PATCH 4/7] hpet: Fix packing for w32: needs packing (bios interface)
>> [PATCH 5/7] usb: Fix packing for w32: needs packing (usb interface)
>> [PATCH 6/7] virtio: Fix packing for w32: needs packing? (guest 
>> interface?)
>> [PATCH 7/7] slirp: Fix packing for w32: needs packing (network interface)
>>
>> All those struct statements need the pack attribute (otherwise the code
>> would have to be rewritten which is of course always possible).
> gesn_cdb in atapi.c, VMDK4Header in vmdk.c and many structures in
> bt.h need be fixed too.

Oops, you are right. Obviously I missed all anonymous structs:
codiff simply ignores them, and pahole must be called with
flags -a -A to show them. Who invented packing of structs?

Comparing the output of pahole -a -A is less elegant than using
codiff, but shows the structs which you mentioned.

I suggest to apply my patch series first because it fixes
the most important bugs in networking. The remaining
bugs are in code which is used less often. They will be
fixed by a second patch series which replaces all remaining
packed attributes.