Re: [Qemu-devel] [PATCH 1/2] KVM: fix i8259 interrupt high to low transition logic

[Jan Kiszka]

On 2012-09-13 15:41, Maciej W. Rozycki wrote:
> On Wed, 12 Sep 2012, Matthew Ogilvie wrote:
> 
>> Also, how big of a concern is a very rare gained or lost IRQ0
>> actually?  Under normal conditions, I would expect this to at most
>> cause a one time clock drift in the guest OS of a fraction of
>> a second.  If that only happens when rebooting or migrating the
>> guest...
> 
>  It depends on how you define "very rare".  Once per month or probably 
> even per day is probably acceptable although you'll see a disruption in 
> the system clock.  This is still likely unwanted if the system is used as 
> a clock reference and not just wants to keep its clock right for own 
> purposes.  Anything more frequent and NTP does care very much; an accurate 
> system clock is important in many uses, starting from basic ones such as 
> where timestamps of files exported over NFS are concerned.
> 
>  Speaking of real hw -- I don't know whether that really matters for 
> emulated systems.  Thanks for looking into the 8254 PIT in details.

First correct, then fast. That rule applies at least to the conceptual
phase. Also, for rarely used PIT modes, I would refrain from optimizing
them away from the specified behaviour.

Jan

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