Re: [qemu-s390x] [Qemu-devel] [PATCH v3 3/3] hw/s390x/ioinst: Fix alignment problem in struct SubchDev

[Peter Maydell]

On Thu, 27 Sep 2018 at 09:25, Thomas Huth <address@hidden> wrote:
>
> struct SubchDev embeds several other structures which are marked with
> QEMU_PACKED. This causes the compiler to not care for proper alignment
> of these structures. When we later pass around pointers to the unaligned
> struct members during migration, this causes problems on host architectures
> like Sparc that can not do unaligned memory access.
>
> Most of the structs in ioinst.h are naturally aligned, so we can fix
> most of the problem by removing the QEMU_PACKED statements (and use
> QEMU_BUILD_BUG_MSG() statements instead to make sure that there is no
> padding). However, for the struct SCHIB, we have to keep the QEMU_PACKED
> since the compiler adds some padding here otherwise. Move this struct
> to the beginning of struct SubchDev instead to fix the alignment problem
> here, too.

Unfortunately clang does not like the struct SCHIB being still
marked packed:

/home/petmay01/linaro/qemu-from-laptop/qemu/hw/s390x/css.c:1294:25:
warning: taking address of packed member 'pmcw' of class or structure
'SCHIB' may result in an unaligned pointer value
[-Waddress-of-packed-member]
    copy_pmcw_to_guest(&dest->pmcw, &src->pmcw);
                        ^~~~~~~~~~
/home/petmay01/linaro/qemu-from-laptop/qemu/hw/s390x/css.c:1294:38:
warning: taking address of packed member 'pmcw' of class or structure
'SCHIB' may result in an unaligned pointer value
[-Waddress-of-packed-member]
    copy_pmcw_to_guest(&dest->pmcw, &src->pmcw);
                                     ^~~~~~~~~
/home/petmay01/linaro/qemu-from-laptop/qemu/hw/s390x/css.c:1295:25:
warning: taking address of packed member 'scsw' of class or structure
'SCHIB' may result in an unaligned pointer value
[-Waddress-of-packed-member]
    copy_scsw_to_guest(&dest->scsw, &src->scsw);
                        ^~~~~~~~~~
/home/petmay01/linaro/qemu-from-laptop/qemu/hw/s390x/css.c:1295:38:
warning: taking address of packed member 'scsw' of class or structure
'SCHIB' may result in an unaligned pointer value
[-Waddress-of-packed-member]
    copy_scsw_to_guest(&dest->scsw, &src->scsw);
                                     ^~~~~~~~~
/home/petmay01/linaro/qemu-from-laptop/qemu/hw/s390x/css.c:1343:27:
warning: taking address of packed member 'pmcw' of class or structure
'SCHIB' may result in an unaligned pointer value
[-Waddress-of-packed-member]
    copy_pmcw_from_guest(&dest->pmcw, &src->pmcw);
                          ^~~~~~~~~~
/home/petmay01/linaro/qemu-from-laptop/qemu/hw/s390x/css.c:1343:40:
warning: taking address of packed member 'pmcw' of class or structure
'SCHIB' may result in an unaligned pointer value
[-Waddress-of-packed-member]
    copy_pmcw_from_guest(&dest->pmcw, &src->pmcw);
                                       ^~~~~~~~~
/home/petmay01/linaro/qemu-from-laptop/qemu/hw/s390x/css.c:1344:27:
warning: taking address of packed member 'scsw' of class or structure
'SCHIB' may result in an unaligned pointer value
[-Waddress-of-packed-member]
    copy_scsw_from_guest(&dest->scsw, &src->scsw);
                          ^~~~~~~~~~
/home/petmay01/linaro/qemu-from-laptop/qemu/hw/s390x/css.c:1344:40:
warning: taking address of packed member 'scsw' of class or structure
'SCHIB' may result in an unaligned pointer value
[-Waddress-of-packed-member]
    copy_scsw_from_guest(&dest->scsw, &src->scsw);
                                       ^~~~~~~~~


Not sure how best to address this. A couple of ideas that I had:

(1) make the 'uint64_t mba' field in the SCHIB struct into
two uint32_t fields, adjusting all the code which needs
to access it accordingly; then we could drop the packed
annotation from the struct

(2) have the guts of copy_{pmcw,scsw}_{to,from}_guest() be
macros, so we can do them inline in the copy_schib_{to,from}_guest()
function and thus operate directly on src->pmcw.foo &c
fields rather than ever having to take the address of any
of the fields in src or dest

thanks
-- PMM