Re: [Minios-devel] [UNIKRAFT PATCH] plat/kvm/x86: retrieve TSC frequency from hypervisor if available

[Simon Kuenzer <simon.kuenzer@xxxxxxxxx>]

Hi Hugo,

nice feature! I got it working by just setting '+invtsc' flag to the cpu 
parameter with QEMU 3.1.0. The vmware-cpuid-freq flag was not required.
Thanks,

Simon

Reviewed-by: Simon Kuenzer <simon.kuenzer@xxxxxxxxx>

On 27.03.20 10:11, Hugo Lefeuvre wrote:
> The TSC clock frequency is currently estimated using the i8254 timer over a
> period of 0.1s. This solution is undesirable because it delays the boot.
>
> Hypervisors advertise the TSC clock frequency via the hypervisor generic
> cpuid timing information leaf 0x40000010 [0]. This feature is available in
> QEMU/KVM since 2.9 with -cpu options vmware-cpuid-freq=on and +invtsc.
>
> Retrieve TSC clock frequency via cpuid. If unavailable, fall back to manual
> calibration.
>
> [0] https://lwn.net/Articles/301888/
>
> Signed-off-by: Hugo Lefeuvre <hugo.lefeuvre@xxxxxxxxx>
> ---
>   plat/kvm/x86/tscclock.c | 34 +++++++++++++++++++++++++---------
>   1 file changed, 25 insertions(+), 9 deletions(-)
>
> diff --git a/plat/kvm/x86/tscclock.c b/plat/kvm/x86/tscclock.c
> index 86db814..727164a 100644
> --- a/plat/kvm/x86/tscclock.c
> +++ b/plat/kvm/x86/tscclock.c
> @@ -216,7 +216,8 @@ __u64 tscclock_monotonic(void)
>    */
>   int tscclock_init(void)
>   {
> -       __u64 tsc_freq, rtc_boot;
> +       __u64 tsc_freq = 0, rtc_boot;
> +       __u32 eax, ebx, ecx, edx;
>   
>   	/* Initialise i8254 timer channel 0 to mode 2 at CONFIG_HZ frequency */
>         outb(TIMER_MODE, TIMER_SEL0 | TIMER_RATEGEN | TIMER_16BIT);
> @@ -230,15 +231,30 @@ int tscclock_init(void)
>         rtc_boot = rtc_gettimeofday();
>   
>   	/*
> -        * Calculate TSC frequency by calibrating against an 0.1s delay
> -        * using the i8254 timer.
> -        * TODO: Find a more elegant solution that does not require us to
> -        * to delay the boot for 100ms. Does KVM provides us a pre-calculated
> -        * TSC value?
> +        * Attempt to retrieve TSC frequency via the hypervisor generic cpuid
> +        * timing information leaf. 0x40000010 returns the (virtual) TSC
> +        * frequency in kHz, or 0 if the feature is not supported by the
> +        * hypervisor.
>          */
> -       tsc_base = rdtsc();
> -       i8254_delay(100000);
> -       tsc_freq = (rdtsc() - tsc_base) * 10;
> +       cpuid(0x40000000, 0, &eax, &ebx, &ecx, &edx);
> +       if (eax >= 0x40000010) {
> +               uk_pr_info("Retrieving TSC clock frequency from hypervisor\n");
> +               cpuid(0x40000010, 0, &eax, &ebx, &ecx, &edx);
> +               tsc_freq = eax * 1000;
> +       }
> +
> +       /*
> +        * If we could not retrieve the TSC frequency from the hypervisor,
> +        * calibrate against an 0.1s delay using the i8254 timer. This is
> +        * undesirable as it delays the boot sequence.
> +        */
> +       if (!tsc_freq) {
> +               uk_pr_info("Calibrating TSC clock against i8254 timer\n");
> +               tsc_base = rdtsc();
> +               i8254_delay(100000);
> +               tsc_freq = (rdtsc() - tsc_base) * 10;
> +       }
> +
>         uk_pr_info("Clock source: TSC, frequency estimate is %llu Hz\n",
>                    (unsigned long long) tsc_freq);
>   
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