linux-kernel - Re: [PATCH 05/10] x86: jailhouse: Set up timekeeping

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Date:   Sat, 18 Nov 2017 20:23:10 +0100
From:   Jan Kiszka <jan.kiszka@....de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>,
        x86@...nel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        jailhouse-dev@...glegroups.com
Subject: Re: [PATCH 05/10] x86: jailhouse: Set up timekeeping

On 2017-11-17 23:49, Thomas Gleixner wrote:
> On Thu, 16 Nov 2017, Jan Kiszka wrote:
>> Calibrate the TSC and, where necessary, the APIC timer against the
>> TMTIMER. We need our own implementation as neither the PIC nor the HPET
>> are available, and the standard calibration routines try to make use of
>> them.
> 
> Why is this needed at all?
> 
> The host the frequency already. So this can be done w/o pmtimer and extra
> calibration routine.

The hypervisor does not have the frequencies. It will never use the APIC
timer (it's owned by the guests), and it has no use case for the TSC so
far. Only the root cell (the Linux that booted the system) has that
data. Now we could

- trust the root cell to provide the right values and export them during
  startup to the hypervisor and from there to the non-root cells.

- calculate the frequencies once and store them in the hyperivsor
  config, just like other system-specific information, for re-export to
  the cells.

But I don't think option 1 will be ok for all use cases. Maybe a
combination of both, falling back to the root cell data if nothing is
defined in the config. Let me think about this.

Jan