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

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Date:   Mon, 20 Nov 2017 13:21:25 +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 2017-11-20 12:24, Thomas Gleixner wrote:
> On Sat, 18 Nov 2017, Jan Kiszka wrote:
>> 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.
> 
> Another question is whether systems which can support jailhouse, have the
> frequencies available via cpuid/msr and can avoid that calibration thing
> completely.

OK, some may (not Xeons, though), and we would not exploit it with this
approach.

Jan