Re: [Qemu-devel] [PATCH v3 1/9] s390x/tcg: avoid overflows in time2tod/tod2time

[Thomas Huth <thuth@redhat.com>]

On 25.06.2018 13:53, David Hildenbrand wrote:
> Big values for the TOD/ns clock can result in some overflows that can be
> avoided. Not all overflows can be handled however, as the conversion either
> multiplies by 4.096 or divided by 4.096.
> 
> Apply the trick used in the Linux kernel in arch/s390/include/asm/timex.h
> for tod_to_ns() and use the same trick also for the conversion in the
> other direction.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  target/s390x/internal.h | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/target/s390x/internal.h b/target/s390x/internal.h
> index e392a02d12..6cf63340bf 100644
> --- a/target/s390x/internal.h
> +++ b/target/s390x/internal.h
> @@ -243,13 +243,14 @@ enum cc_op {
>  /* Converts ns to s390's clock format */
>  static inline uint64_t time2tod(uint64_t ns)
>  {
> -    return (ns << 9) / 125;
> +    return (ns << 9) / 125 + (((ns & 0xff10000000000000ull) / 125) << 9);
> +
>  }
>  
>  /* Converts s390's clock format to ns */
>  static inline uint64_t tod2time(uint64_t t)
>  {
> -    return (t * 125) >> 9;
> +    return ((t >> 9) * 125) + (((t & 0x1ff) * 125) >> 9);
>  }
>  
>  static inline hwaddr decode_basedisp_s(CPUS390XState *env, uint32_t ipb,
> 

Looks reasonable.

Reviewed-by: Thomas Huth <thuth@redhat.com>