RE: [PATCH] x86/hyperv: Pass on the lpj value from host to guest

[Michael Kelley (LINUX)]

From: Stanislav Kinsburskii <skinsburskii@xxxxxxxxxxxxxxxxxxx>
> 
> And have it preset.
> This change allows to significantly reduce time to bring up guest SMP
> configuration as well as make sure the guest won't get inaccurate
> calibration results due to "noisy neighbour" situation.
> 
> Below are the numbers for 16 VCPU guest before the patch (~1300 msec)
> 
> [    0.562938] x86: Booting SMP configuration:
> ...
> [    1.859447] smp: Brought up 1 node, 16 CPUs
> 
> and after the patch (~130 msec):
> 
> [    0.445079] x86: Booting SMP configuration:
> ...
> [    0.575035] smp: Brought up 1 node, 16 CPUs
> 
> This change is inspired by commit 0293615f3fb9 ("x86: KVM guest: use
> paravirt function to calculate cpu khz").

This patch has been nagging at me a bit, and I finally did some further
checking.   Looking at Linux guests on local Hyper-V and in Azure, I see
a dmesg output line like this during boot:

Calibrating delay loop (skipped), value calculated using timer frequency.. 5187.81 BogoMIPS (lpj=2593905)

We're already skipping the delay loop calculation because lpj_fine
is set in tsc_init(), using the results of get_loops_per_jiffy().  The
latter does exactly the same calculation as hv_preset_lpj() in
this patch.

Is this patch arising from an environment where tsc_init() is
skipped for some reason?  Just trying to make sure we fully
when this patch is applicable, and when not.

Michael

> 
> Signed-off-by: Stanislav Kinsburskiy <stanislav.kinsburskiy@xxxxxxxxx>
> CC: "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>
> CC: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
> CC: Wei Liu <wei.liu@xxxxxxxxxx>
> CC: Dexuan Cui <decui@xxxxxxxxxxxxx>
> CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> CC: Ingo Molnar <mingo@xxxxxxxxxx>
> CC: Borislav Petkov <bp@xxxxxxxxx>
> CC: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> CC: x86@xxxxxxxxxx
> CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
> CC: linux-hyperv@xxxxxxxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  arch/x86/kernel/cpu/mshyperv.c |   21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index dedec2f23ad1..0282b2e96cc2 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -320,6 +320,21 @@ static void __init hv_smp_prepare_cpus(unsigned int
> max_cpus)
>  }
>  #endif
> 
> +static void __init __maybe_unused hv_preset_lpj(void)
> +{
> +	unsigned long khz;
> +	u64 lpj;
> +
> +	if (!x86_platform.calibrate_tsc)
> +		return;
> +
> +	khz = x86_platform.calibrate_tsc();
> +
> +	lpj = ((u64)khz * 1000);
> +	do_div(lpj, HZ);
> +	preset_lpj = lpj;
> +}
> +
>  static void __init ms_hyperv_init_platform(void)
>  {
>  	int hv_max_functions_eax;
> @@ -521,6 +536,12 @@ static void __init ms_hyperv_init_platform(void)
> 
>  	/* Register Hyper-V specific clocksource */
>  	hv_init_clocksource();
> +
> +	/*
> +	 * Preset lpj to make calibrate_delay a no-op, which is turn helps to
> +	 * speed up secondary cores initialization.
> +	 */
> +	hv_preset_lpj();
>  #endif
>  	/*
>  	 * TSC should be marked as unstable only after Hyper-V
>