Re: [Qemu-devel] [PATCH] hw/s390/ccw.c: Don't take address of packed members

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 10 Dec 2018 13:58:03 +0000
Peter Maydell <peter.maydell@linaro.org> wrote:

> Taking the address of a field in a packed struct is a bad idea, because
> it might not be actually aligned enough for that pointer type (and
> thus cause a crash on dereference on some host architectures). Newer
> versions of clang warn about this.
> 
> Avoid the problem by using local copies of the PMCW and SCSW
> struct fields in copy_schib_from_guest() and copy_schib_to_guest().
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> This seemed like a not totally ugly and reasonably localised fix
> that satisfies clang. Oddly, this makes the generated object file
> 15K smaller (421K vs 406K), so it might even be better code...

Nice :)

> 
>  hw/s390x/css.c | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/s390x/css.c b/hw/s390x/css.c
> index 04ec5cc9705..ef07691e36b 100644
> --- a/hw/s390x/css.c
> +++ b/hw/s390x/css.c
> @@ -1290,9 +1290,15 @@ void copy_scsw_to_guest(SCSW *dest, const SCSW *src)
>  static void copy_schib_to_guest(SCHIB *dest, const SCHIB *src)
>  {
>      int i;
> +    PMCW srcpmcw, destpmcw;
> +    SCSW srcscsw, destscsw;

<bikeshed>
I would find src_pmcw etc. easier to read. Other opinions?
</bikeshed>

>  
> -    copy_pmcw_to_guest(&dest->pmcw, &src->pmcw);
> -    copy_scsw_to_guest(&dest->scsw, &src->scsw);
> +    srcpmcw = src->pmcw;
> +    copy_pmcw_to_guest(&destpmcw, &srcpmcw);
> +    dest->pmcw = destpmcw;
> +    srcscsw = src->scsw;
> +    copy_scsw_to_guest(&destscsw, &srcscsw);
> +    dest->scsw = destscsw;
>      dest->mba = cpu_to_be64(src->mba);
>      for (i = 0; i < ARRAY_SIZE(dest->mda); i++) {
>          dest->mda[i] = src->mda[i];
> @@ -1339,9 +1345,15 @@ static void copy_scsw_from_guest(SCSW *dest, const SCSW *src)
>  static void copy_schib_from_guest(SCHIB *dest, const SCHIB *src)
>  {
>      int i;
> +    PMCW srcpmcw, destpmcw;
> +    SCSW srcscsw, destscsw;
>  
> -    copy_pmcw_from_guest(&dest->pmcw, &src->pmcw);
> -    copy_scsw_from_guest(&dest->scsw, &src->scsw);
> +    srcpmcw = src->pmcw;
> +    copy_pmcw_from_guest(&destpmcw, &srcpmcw);
> +    dest->pmcw = destpmcw;
> +    srcscsw = src->scsw;
> +    copy_scsw_from_guest(&destscsw, &srcscsw);
> +    dest->scsw = destscsw;
>      dest->mba = be64_to_cpu(src->mba);
>      for (i = 0; i < ARRAY_SIZE(dest->mda); i++) {
>          dest->mda[i] = src->mda[i];