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

[Cornelia Huck]

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.

> +    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?