Re: [dpdk-dev] [PATCH v2 2/2] timer: support EAL functions on Windows

[Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>]

On 2020-04-27 15:20 GMT+0300 Fady Bader wrote:
> Implemented the needed Windows eal timer functions.
[snip]
> +void
> +rte_delay_us_sleep(unsigned int us)
> +{
> +	HANDLE timer;
> +	LARGE_INTEGER li_due_time;

Here usually comes a blank line.

> +	/* create waitable timer */
> +	timer = CreateWaitableTimer(NULL, TRUE, NULL);
> +	if (!timer) {
> +		RTE_LOG_WIN32_ERR("CreateWaitableTimer()");
> +		rte_errno = ENOMEM;
> +		return;
> +	}
> +
> +	/* set us microseconds time for timer */
> +	li_due_time.QuadPart = -(us * 10);

The comment is still misleading.

[snip]
> +uint64_t
> +get_tsc_freq(void)
> +{
> +	uint64_t tsc_freq;
> +	LARGE_INTEGER Frequency;
> +
> +	QueryPerformanceFrequency(&Frequency);
> +
> +	/*
> +	 * Mulitply by 1K to obtain the true frequency of the CPU
> +	 * it was noted in the MSDN "in many cases, QueryPerformanceFrequency
> +	 * returns the TSC frequency divided by 1024"
> +	 */
> +	tsc_freq = ((uint64_t)Frequency.QuadPart * 1024);
> +
> +	return tsc_freq;
> +}

Extended quote from MSDN:

	Cases might exist where QueryPerformanceFrequency doesn't return the
	actual frequency of the hardware tick generator. For example, in many
	cases, QueryPerformanceFrequency returns the TSC frequency divided by
	1024; and on Hyper-V, the performance counter frequency is always 10
	MHz when the guest virtual machine runs under a hypervisor that
	implements the hypervisor version 1.0 interface. As a result, don't
	assume that QueryPerformanceFrequency will return the precise TSC
	frequency.

	https://docs.microsoft.com/en-us/windows/win32/sysinfo/acquiring-high-resolution-time-stamps

"In many cases" is pretty vague, we can't distinguish such cases.
Here's what I observe on my QEMU guest ("ticks" is Win32 API, "tsc" is RDTSC):

freq = 100000000 Hz        # note: 100 MHz, not 10 MHz
delta (ticks) = 100401071  # Sleep(1000)
delta (secs)  = 1.004011   # delta (ticks) / freq
delta (tsc) = 3460948332   # roughly CPU clock frequency
tsc / tick  = 34.471229

I suggest measuring a real-time delay with rte_get_tsc_cycles() from arch/
as other EALs do when HPET is not available or configured. This is
discouraged by MSDN, but its reasons seem irrelevant for DPDK.

-- 
Dmitry Kozlyuk