From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <SRS0=VxZE=JO=vger.kernel.org=linux-kernel-owner@kernel.org>
Date: Thu, 28 Jun 2018 13:46:15 +0200
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v12 09/11] x86/tsc: prepare for early sched_clock
Message-ID: <20180628114615.GF2494@hirez.programming.kicks-ass.net>
References: <20180621212518.19914-1-pasha.tatashin@oracle.com>
 <20180621212518.19914-10-pasha.tatashin@oracle.com>
 <alpine.DEB.2.21.1806231815060.8650@nanos.tec.linutronix.de>
 <alpine.DEB.2.21.1806232210110.8650@nanos.tec.linutronix.de>
 <CAGM2reYQx769omkWVweEcTMy8WNrKOiuUVo4OFWNh4zW-rc4pg@mail.gmail.com>
 <alpine.DEB.2.21.1806232347540.8650@nanos.tec.linutronix.de>
 <alpine.DEB.2.21.1806261725510.1587@nanos.tec.linutronix.de>
 <CAGM2reYQ23XhQHAOO0Oo8MGvgrjpNZPZkwp+wwsU8Fb+jfANyA@mail.gmail.com>
 <alpine.DEB.2.21.1806270947410.1587@nanos.tec.linutronix.de>
 <alpine.DEB.2.21.1806281159590.1778@nanos.tec.linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <alpine.DEB.2.21.1806281159590.1778@nanos.tec.linutronix.de>
Sender: linux-kernel-owner@vger.kernel.org
List-Archive: <https://lore.kernel.org/lkml/>
List-Post: <mailto:linux-kernel@vger.kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Pavel Tatashin <pasha.tatashin@oracle.com>, Steven Sistare <steven.sistare@oracle.com>, Daniel Jordan <daniel.m.jordan@oracle.com>, linux@armlinux.org.uk, Martin Schwidefsky <schwidefsky@de.ibm.com>, Heiko Carstens <heiko.carstens@de.ibm.com>, John Stultz <john.stultz@linaro.org>, sboyd@codeaurora.org, x86@kernel.org, LKML <linux-kernel@vger.kernel.org>, mingo@redhat.com, "H. Peter Anvin" <hpa@zytor.com>, douly.fnst@cn.fujitsu.com, Prarit Bhargava <prarit@redhat.com>, feng.tang@intel.com, Petr Mladek <pmladek@suse.com>, gnomes@lxorguk.ukuu.org.uk, linux-s390@vger.kernel.org, Andy Shevchenko <andriy.shevchenko@linux.intel.com>, Boris Ostrovsky <boris.ostrovsky@oracle.com>
List-ID: <linux-s390.vger.kernel.org>

On Thu, Jun 28, 2018 at 12:43:59PM +0200, Thomas Gleixner wrote:
> 	init_hypervisor_platform()
> 	   vmware:
> 	           Retrieves frequency and store it for the
> 		   calibration function
> 
> 		   khz = vmware_get_khz_magic()
> 		   vmware_tsc_khz = khz
> 		   calibrate_cpu = vmware_get_tsc_khz
> 	   	   calibrate_tsc = vmware_get_tsc_khz
> 		   preset_lpj(khz)
> 
>  	   hyperv:
> 		   if special hyperv MSRs are available:
> 
> 		      calibrate_cpu = hv_get_tsc_khz
> 		      calibrate_tsc = hv_get_tsc_khz
> 
> 		   MSR is readable already in this function
> 
> 	   jailhouse:
> 	   
> 		   Frequency is available in this function and store
> 		   in a variable for the calibration function
> 
> 		   calibrate_cpu	= jailhouse_get_tsc
> 		   calibrate_tsc	= jailhouse_get_tsc
> 
> 	...
> 	
> 	kvmclock_init()
> 
> 		if (magic_conditions)
> 			calibrate_tsc = kvm_get_tsc_khz
> 			calibrate_cpu = kvm_get_tsc_khz
> 
> 			kvm_get_preset_lpj()
> 			   khz = kvm_get_tsc_khz()
> 			   preset_lpj(khz);

Note that all these which get TSC values from a HV _should_ set
X86_FEATURE_TSC_KNOWN_FREQ to avoid the late recalibrate.

Calibrating against a virtual PIT/HPET/PMTIMER is utterly pointless.

> The generic initilizaiton does everything twice, which makes no sense,
> except for the unlikely case were no fast functions are available and the
> quick PIT calibration fails (PMTIMER/HPET) are not available in early
> calibration. HPET 

Incomplete; but I suspect you want to talk about how we can make HPET
available early by putting it in a fixmap.

And only if we fail, do we at a later stage try again using PMTIMER.

Currently it all works by accident, since !hpet and acpi_pm_read_early()
returns 0, but really we should not be running the fallback crap at all
that early.