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Re: [Xen-devel] [PATCH 2/4] x86/time: reduce rounding errors in calculations
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 13 Mar 2020 15:14:55 +0000
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- Cc: Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Fri, 13 Mar 2020 15:15:19 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 13/03/2020 09:25, Jan Beulich wrote:
> Plain (unsigned) integer division simply truncates the results. The
> overall errors are smaller though if we use proper rounding. (Extend
> this to the purely cosmetic aspect of time.c's freq_string(), which
> before this change I've frequently observed to report e.g. NN.999MHz
> HPET clock speeds.)
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> We could switch at least the first div/rem pair in calibrate_APIC_clock()
> to use do_div(), but this would imply switching bus_freq (and then also
> result) to unsigned int (as the divisor has to be 32-bit). While I think
> there's pretty little risk for bus frequencies to go beyond 4GHz in the
> next so many years, I still wasn't certain enough this would be a welcome
> change.
Honestly, do_div() is very easy to get wrong (and in security relevant
ways in Linux). I'd advocate for phasing its use out, irrespective of
this frequency concern.
As for 4GHz, I think its unlikely, but better to be safe in the code.
>
> --- a/xen/arch/x86/apic.c
> +++ b/xen/arch/x86/apic.c
> @@ -1261,7 +1261,9 @@ static int __init calibrate_APIC_clock(v
> /* set up multipliers for accurate timer code */
> bus_freq = result*HZ;
> bus_cycle = (u32) (1000000000000LL/bus_freq); /* in pico seconds */
> + bus_cycle += (1000000000000UL % bus_freq) * 2 > bus_freq;
These two differ in signedness of the numerator. GCC seems to cope with
combining the two into a single div instruction, but I think we should
be consistent with the constant nevertheless.
Otherwise, Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
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