Re: [Xen-devel] [PATCH v5 2/5] x86/hpet: Use singe apic vector rather than irq_descs for HPET interrupts

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.03.14 at 16:43, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>  ** If an HPET firing at an appropriate time can be found (up to 20us late), a
>     CPU will simply request to be woken up with that HPET.

With exit latencies from C1/C1E in the range of 1...10us, 20us seems
like a lot of additional latency added here.

>  ** Failing finding an appropriate timed HPET, a CPU shall find the soonest
>     late HPET and program it earlier.
>  ** Failing any late HPETs, a CPU shall wake up with the latest early HPET it
>     can find.

And do what?

>  ** Failing all else, a CPU shall retry to find a free HPET.  This guarantees
>     that a CPU will never leave hpet_broadcast_enter() without arranging an
>     interrupt.

For how long? Indefinitely (i.e. until the wakeup time is reached)?

All without having looked at the details of the patch yet.

Jan


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