[Qemu-devel] Re: [PATCH 0/5] Refactor and enhance RTC configuration

[Jan Kiszka <jan.kiszka@web.de>]

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Blue Swirl wrote:
> On Wed, Sep 9, 2009 at 6:11 PM, Jan Kiszka<jan.kiszka@web.de> wrote:
>> The aim of this series is to allow using the emulated PC RTC (MC146818)
>> as a reliable time source for guests. This is particularly useful if the
>> host runs NTP or has otherwise access to an accurate clock while the
>> guest has not (no network, impossible to add an NTP implementation
>> etc.).
> 
> What I meant in the earlier thread is that m48t59 port read for time
> or date gets the data directly from host using  qemu_get_timedate().
> PC RTC instead uses a timer: the timer callback updates the cmos_data
> structure. It seems that you just replace the timer with a new one.
> I'd remove the timer altogether and change the cmos_ioport_read to
> call qemu_get_timedate().

That was my first idea as well, but the timer is there to emulate real
hardware behavior: Updates only take place while the UIP flag is set in
register A, and they take a few hundred microseconds. Guests actually
wait for this to happen and only read out after such a change. Moreover,
updates can trigger interrupts.

For sure one could avoid these update timers. But as they are so rare (1
HZ...), I didn't want to prematurely optimize the code here, risking
regressions while trying to fix our problems.

Jan


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