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


From: Thomas Huth
Subject: Re: [qemu-s390x] [PATCH v2 2/3] hw/s390x/css: Remove QEMU_PACKED from struct SenseId
Date: Wed, 26 Sep 2018 10:09:17 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

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.

 Thomas





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