From mboxrd@z Thu Jan  1 00:00:00 1970
From: Zachary Amsden <zach@vmware.com>
Subject: Re: + stupid-hack-to-make-mainline-build.patch added to -mm tree
Date: Tue, 06 Mar 2007 00:55:30 -0800
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To: Ingo Molnar <mingo@elte.hu>
Cc: Virtualization Mailing List <virtualization@lists.osdl.org>, mm-commits@vger.kernel.org, tglx@linutronix.de, akpm@linux-foundation.org
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Ingo Molnar wrote:
> * Ingo Molnar <mingo@elte.hu> wrote:
>
>   =

>> no, that's not the case: next_timer_interrupt() is the NO_IDLE_HZ =

>> method of doing things - while in the NO_HZ case you are supposed to =

>> use clockevent devices to program timer hardware.
>>     =


We don't have a clockevent device.  But we need NO_IDLE_HZ support, =

which NO_HZ has now subsumed.

> a proper CE device also has the added bonus of making high-res timers =

> guests work automatically. It should be simple: just pass it through to =

> your hypervisor, a hyper-CE-device, like a hyper-clocksource device has =

> essentially no guest-side complexity.
>   =


It is not so simple.  In theory it works great.  In reality, the i386 =

implementation is completely hardwired to work the way hardware works, =

and breaking the clockevent code out of the deep ties to the APIC is =

extremely non-trivial.  We tried, and could not accomplish it for 2.6.21 =

because the hrtimers integration was complex, and introduced many bugs =

for us.  We worked around this by keeping NO_IDLE_HZ support, which now =

you deprecated.  So now we are using NO_HZ without a hyper-CE device, =

and it is working fine.  We understand the benefits of moving to the CE =

model - but it cannot be done overnight.

Xen has the same requirements for integrating their timer code.

Zach