Re: [Minios-devel] [UNIKRAFT PATCH 3/3] plat/kvm: Add KVM (x86_64) timer support

[Wei Chen <Wei.Chen@xxxxxxx>]

Hi Costin,

> -----Original Message-----
> From: Minios-devel <minios-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of
> Costin Lupu
> Sent: 2018年5月1日 7:55
> To: Simon Kuenzer <simon.kuenzer@xxxxxxxxx>; minios-devel@xxxxxxxxxxxxx
> Subject: Re: [Minios-devel] [UNIKRAFT PATCH 3/3] plat/kvm: Add KVM (x86_64)
> timer support
> 
> On 04/30/2018 03:44 PM, Simon Kuenzer wrote:
> > See my comments inline.
> >
> > On 05.04.2018 17:21, Costin Lupu wrote:
> >> We are using TSC clock as main timer on KVM.
> >>
> >> Signed-off-by: Costin Lupu <costin.lupu@xxxxxxxxx>
> >> ---
> >>   plat/kvm/Makefile.uk              |   3 +
> >>   plat/kvm/clock_subr.c             | 226 ++++++++++++++++++++++++
> >>   plat/kvm/include/kvm/clock_subr.h |  83 +++++++++
> >>   plat/kvm/include/kvm/tscclock.h   |  42 +++++
> >>   plat/kvm/irq.c                    |  10 ++
> >>   plat/kvm/time.c                   |  62 +++++++
> >>   plat/kvm/tscclock.c               | 356


[...] trimming for easy reading

> >> +/*
> >> + * Calibrate TSC and initialise TSC clock.
> >> + */
> >> +int tscclock_init(void)
> >> +{
> >> +    __u64 tsc_freq, rtc_boot;
> >> +
> >> +    /* Initialise i8254 timer channel 0 to mode 2 at 100 Hz */
> >> +    outb(TIMER_MODE, TIMER_SEL0 | TIMER_RATEGEN | TIMER_16BIT);
> >> +    outb(TIMER_CNTR, (TIMER_HZ / 100) & 0xff);
> >> +    outb(TIMER_CNTR, (TIMER_HZ / 100) >> 8);
> >> +
> >> +    /*
> >> +     * Read RTC "time at boot". This must be done just before
> >> tsc_base is
> >> +     * initialised in order to get a correct offset below.
> >> +     */
> >> +    rtc_boot = rtc_gettimeofday();
> >> +
> >> +    /*
> >> +     * Calculate TSC frequency by calibrating against an 0.1s delay
> >> +     * using the i8254 timer.
> >> +     */
> >
> > Wow, this is adds a 100ms boot delay to the Unikernels on KVM. Can you
> > put an TODO comment for revisiting this later? Maybe we can find a
> > different method to get the correct value for the TSC frequency.
> 
> Right.
> 

It seems that QEMU-KVM publish TSC frequency to the guest OS in CPUID page
0x40000010. Can we read the tsc_freq from CPUID just like VMWare does?

> >> +    tsc_base = rdtsc();
> >> +    i8254_delay(100000);
> >> +    tsc_freq = (rdtsc() - tsc_base) * 10;
> >> +    uk_printd(DLVL_INFO,
> >> +        "Clock source: TSC, frequency estimate is %llu Hz\n",
> >> +        (unsigned long long) tsc_freq); //TODO
> >> +
> >> +    /*
> >> +     * Calculate TSC scaling multiplier.
> >> +     *
> >> +     * (0.32) tsc_mult = NSEC_PER_SEC (32.32) / tsc_freq (32.0)
> >> +     */
> >> +    tsc_mult = (NSEC_PER_SEC << 32) / tsc_freq;
> >> +
> >> +    /*
> >> +     * Monotonic time begins at tsc_base (first read of TSC before
> >> +     * calibration).
> >> +     */
> >> +    time_base = mul64_32(tsc_base, tsc_mult);
> >> +
> >> +    /*
> >> +     * Compute RTC epoch offset by subtracting monotonic time_base
> >> from RTC
> >> +     * time at boot.
> >> +     */
> >> +    rtc_epochoffset = rtc_boot - time_base;
> >> +
> >> +    /*
> >> +     * Initialise i8254 timer channel 0 to mode 4 (one shot).
> >> +     */
> >> +    outb(TIMER_MODE, TIMER_SEL0 | TIMER_ONESHOT | TIMER_16BIT);
> >> +
> >> +    return 0;
> >> +}

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