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Subject: Re: [Qemu-devel] [PATCH v1 7/7] kvm/x86: Hyper-V SynIC timers
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To: Roman Kagan <rkagan@virtuozzo.com>, Andrey Smetanin <asmetanin@virtuozzo.com>, kvm@vger.kernel.org, Gleb Natapov <gleb@kernel.org>, "K. Y. Srinivasan" <kys@microsoft.com>, Haiyang Zhang <haiyangz@microsoft.com>, Vitaly Kuznetsov <vkuznets@redhat.com>, "Denis V. Lunev" <den@openvz.org>, qemu-devel@nongnu.org



On 27/11/2015 09:12, Roman Kagan wrote:
>> > +	n = div64_u64(time_now - stimer->exp_time, stimer->count) + 1;
>> > +	stimer->exp_time += n * stimer->count;
> This is actually just a reminder calculation so I'd rather do it
> directly with div64_u64_rem().

It took me a while to understand why it was a remained. :)  Expanding
Andrey's formula you get

	exp_time = exp_time + count + (time_now - exp_time) / count * count

the remainder, (time_now - exp_time) % count would be

	time_now - exp_time - (time_now - exp_time) / count * count

so -((time_now - exp_time) % count) is

	exp_time - time_now + (time_now - exp_time) / count * count

so Andrey's expression is

	exp_time = time_now + (count - (time_now - exp_time) % count)

Yeah, that looks nice.

Paolo