Re: [PATCH 18/18] Add timekeeping documentation

[Zachary Amsden <zamsden@redhat.com>]

On 07/13/2010 09:16 PM, Takuya Yoshikawa wrote:
> Hi,
>
> (2010/07/13 11:25), Zachary Amsden wrote:
>> +
>> +2.3) APIC
>> +
>> +On Pentium and later processors, an on-board timer is available to 
>> each CPU
>> +as part of the Advanced Programmable Interrupt Controller.  The APIC is
>> +accessed through memory-mapped registers and provides interrupt 
>> service to each
>> +CPU, used for IPIs and local timer interrupts.
>> +
>> +Although in theory the APIC is a safe and stable source for local 
>> interrupts,
>> +in practice, many bugs and glitches have occurred due to the special 
>> nature of
>> +the APIC CPU-local memory-mapped hardware.  Beware that CPU errata 
>> may affect
>> +the use of the APIC and that workarounds may be required.  In 
>> addition, some of
>> +these workarounds pose unique constraints for virtualization - 
>> requiring either
>> +extra overhead incurred from extra reads of memory-mapped I/O or 
>> additional
>> +functionality that may be more computationally expensive to implement.
>> +
>> +Since the APIC is documented quite well in the Intel and AMD 
>> manuals, we will
>> +avoid repititon of the detail here.  It should be pointed out that 
>> the APIC
>
>          repetition?
>
>> +timer is programmed through the LVT (local vector timer) register, 
>> is capable
>> +of one-shot or periodic operation, and is based on the bus clock 
>> divided down
>> +by the programmable divider register.
>> +
>> +2.4) HPET
>> +
>> +HPET is quite complex, and was originally intended to replace the 
>> PIT / RTC
>> +support of the X86 PC.  It remains to be seen whether that will be 
>> the case, as
>> +the de facto standard of PC hardware is to emulate these older 
>> devices.  Some
>> +systems designated as legacy free may support only the HPET as a 
>> hardware timer
>> +device.
>> +
>> +The HPET spec is rather loose and vague, requiring at least 3 
>> hardware timers,
>> +but allowing implementation freedom to support many more.  It also 
>> imposes no
>> +fixed rate on the timer frequency, but does impose some extremal 
>> values on
>> +frequency, error and slew.
>> +
>> +In general, the HPET is recommended as a high precision (compared to 
>> PIT /RTC)
>> +time source which is independent of local variation (as there is 
>> only one HPET
>> +in any given system).  The HPET is also memory-mapped, and its 
>> presence is
>> +indicated through ACPI tables by the BIOS.
>> +
>> +Detailed specification of the HPET is beyond the current scope of this
>> +document, as it is also very well documented elsewhere.
>> +
>
>
>> +3.6) TSC and STPCLK / T-states
>> +
>> +External signals given to the processor may also have the affect of 
>> stopping
>
>                                                              effect?
>
>> +the TSC.  This is typically done for thermal emergency power control 
>> to prevent
>> +an overheating condition, and typically, there is no way to detect 
>> that this
>> +condition has happened.
>> +
>
>
>> +4.4) Migration
>> +
>> +Migration of a virtual machine raises problems for timekeeping in 
>> two ways.
>> +First, the migration itself may take time, during which interrupts 
>> cannot be
>> +delivered, and after which, the guest time may need to be caught 
>> up.  NTP may
>> +be able to help to some degree here, as the clock correction 
>> required is
>> +typically small enough to fall in the NTP-correctable window.
>> +
>> +An additional concern is that timers based off the TSC (or HPET, if 
>> the raw bus
>> +clock is exposed) may now be running at different rates, requiring 
>> compensation
>> +in some may in the hypervisor by virtualizing these timers.  In 
>> addition,
>
>            way?
>
>> +migrating to a faster machine may preclude the use of a passthrough 
>> TSC, as a
>> +faster clock cannot be made visible to a guest without the potential 
>> of time
>> +advancing faster than usual.  A slower clock is less of a problem, 
>> as it can
>> +always be caught up to the original rate.  KVM clock avoids these 
>> problems by
>> +simply storing multipliers and offsets gainst the TSC for the guest 
>> to convert
>
>                                           against?
>
>> +back into nanosecond resolution values.
>> +
>
>
>   Takuya
>
>     -- I'm not English speaker, so not so sure about some places.

Thanks, you found some mistakes anyway, will fix.

Cheers,

Zach