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

[Thomas Huth]

On 2018-09-26 11:53, Cornelia Huck wrote:
> On Wed, 26 Sep 2018 09:38:46 +0200
> Thomas Huth <address@hidden> 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;
> 
> The doc states that byte 7 always consists of zeroes... but copying the
> 'unused' field is probably less magic than just writing 0.

At least this is also what the previous code was doing. I don't think we
should change it in this patch here.

>> +    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);
>>      }
>>  }
> 
> It seems our only choice is which kind of ugly we prefer when fixing
> this issue... at least the usage of stw_be_p makes this look a bit
> better :)
> 
> But maybe add a comment
> 
> /*
>  * As the SenseId struct cannot be packed (would cause unaligned
>  * accesses), we have to copy the individual fields to an unstructured
>  * area using the correct layout.
>  */
> 
> so that we don't wonder why it looks like this in the future?

Sure, could you add it when picking up the patch? Or shall I respin?

 Thomas