Re: [qemu-s390x] [PATCH v2 2/3] hw/s390x/css: Remove QEMU_PACKED from struct SenseId

[Thomas Huth]

On 2018-09-26 10:17, David Hildenbrand wrote:
> On 26/09/2018 10:09, Thomas Huth wrote:
>> On 2018-09-26 10:07, David Hildenbrand wrote:
>>> On 26/09/2018 10:04, David Hildenbrand wrote:
>>>> On 26/09/2018 09:38, Thomas Huth wrote:
>>>>> The uint16_t member cu_type of struct SenseId is not naturally aligned,
>>>>> and since the struct is marked with QEMU_PACKED, this can lead to
>>>>> unaligned memory accesses - which does not work on architectures like
>>>>> Sparc. Thus remove the QEMU_PACKED here and rather copy the struct
>>>>> byte by byte when we do copy_sense_id_to_guest().
>>>>>
>>>>> Signed-off-by: Thomas Huth <address@hidden>
>>>>> ---
>>>>>  hw/s390x/css.c         | 33 +++++++++++++++++----------------
>>>>>  include/hw/s390x/css.h |  2 +-
>>>>>  2 files changed, 18 insertions(+), 17 deletions(-)
>>>>>
>>>>> diff --git a/hw/s390x/css.c b/hw/s390x/css.c
>>>>> index 5a9fe45..0e51b85 100644
>>>>> --- a/hw/s390x/css.c
>>>>> +++ b/hw/s390x/css.c
>>>>> @@ -750,20 +750,20 @@ static void sch_handle_halt_func(SubchDev *sch)
>>>>>  
>>>>>  }
>>>>>  
>>>>> -static void copy_sense_id_to_guest(SenseId *dest, SenseId *src)
>>>>> +static void copy_sense_id_to_guest(uint8_t *dest, SenseId *src)
>>>>>  {
>>>>>      int i;
>>>>>  
>>>>> -    dest->reserved = src->reserved;
>>>>> -    dest->cu_type = cpu_to_be16(src->cu_type);
>>>>> -    dest->cu_model = src->cu_model;
>>>>> -    dest->dev_type = cpu_to_be16(src->dev_type);
>>>>> -    dest->dev_model = src->dev_model;
>>>>> -    dest->unused = src->unused;
>>>>> -    for (i = 0; i < ARRAY_SIZE(dest->ciw); i++) {
>>>>> -        dest->ciw[i].type = src->ciw[i].type;
>>>>> -        dest->ciw[i].command = src->ciw[i].command;
>>>>> -        dest->ciw[i].count = cpu_to_be16(src->ciw[i].count);
>>>>> +    dest[0] = src->reserved;
>>>>> +    stw_be_p(dest + 1, src->cu_type);
>>>>> +    dest[3] = src->cu_model;
>>>>> +    stw_be_p(dest + 4, src->dev_type);
>>>>> +    dest[6] = src->dev_model;
>>>>> +    dest[7] = src->unused;
>>>>> +    for (i = 0; i < ARRAY_SIZE(src->ciw); i++) {
>>>>> +        dest[8 + i * 4] = src->ciw[i].type;
>>>>> +        dest[9 + i * 4] = src->ciw[i].command;
>>>>> +        stw_be_p(dest + 10 + i * 4, src->ciw[i].count);
>>>>
>>>>
>>>> Not really a fan of this, as we sacrifice readability due to one
>>>> unaligned member. What about only converting the unaligned members (e.g.
>>>> cu_type) from uint16_t to uint8_t[2] and adding a comment why this is
>>>> split. Then the structure is naturally packed.
>>>>
>>>> We only have to fixup the places that check cu_type.
>>>>
>>>
>>> Just realized this was basically suggested by Peter. If it would be as
>>> simple as splitting VMSTATE_UINT16 into two VMSTATE_UINT8 or similar, I
>>> would prefer that.
>>
>> It's not that simple, it would break migration from older versions of
>> QEMU due to endianness issues then.
> 
> Migration between different QEMUs (e.g. big to little) is not supported
> as far as I remember. But my head always hurts when thinking about
> endianness conversions, so I am pretty sure I am missing something here.

I was not talking about migration between hosts with different
endianess, but e.g. migration from a x86 host to a x86 host. If you want
to send 0x1234, that would be 0x34 0x12 when using a 16-bit value, but
if you break it up into hi- and low, then it's 0x12 0x34 instead.

Hmm, actually the migration code seems to properly convert 16-bit values
to network byte order, so maybe this could even work. But honestly, I
still think we should avoid QEMU_PACKED as much as possible and better
fill in the memory in copy_sense_id_to_guest() via a byte array here. As
we've seen now, QEMU_PACKED can easily result in non-portable code, so
even if copy_sense_id_to_guest() looks a little bit uglier now than
before, it's certainly the better and more portable way to do this.

 Thomas