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Date: Tue, 27 Mar 2012 12:48:06 +0200
From: Gleb Natapov <gleb@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH uq/master] kvm: Drop unused
	kvm_pit_in_kernel
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To: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>, Avi Kivity <avi@redhat.com>, kvm <kvm@vger.kernel.org>, qemu-devel <qemu-devel@nongnu.org>

On Thu, Mar 22, 2012 at 02:27:37PM +0100, Jan Kiszka wrote:
> On 2012-03-22 13:52, Gleb Natapov wrote:
> > On Thu, Mar 22, 2012 at 01:41:18PM +0100, Jan Kiszka wrote:
> >> On 2012-03-22 08:18, Gleb Natapov wrote:
> >>> On Wed, Mar 21, 2012 at 02:49:09PM +0100, Jan Kiszka wrote:
> >>>> On 2012-03-21 14:41, Gleb Natapov wrote:
> >>>>> On Wed, Mar 21, 2012 at 02:39:47PM +0100, Jan Kiszka wrote:
> >>>>>> On 2012-03-21 14:36, Avi Kivity wrote:
> >>>>>>> On 03/21/2012 02:36 PM, Jan Kiszka wrote:
> >>>>>>>> This is now implied by kvm_irqchip_in_kernel.
> >>>>>>>>
> >>>>>>>>
> >>>>>>>
> >>>>>>> So we can't have -no-kvm-pit?
> >>>>>>>
> >>>>>>> No huge loss, but unexpected.
> >>>>>>
> >>>>>> See e81dda195556e72f8cd294998296c1051aab30a8.
> >>>>>>
> >>>>> I am curious what is the reason for upstream to not supporting disabling the
> >>>>> in-kernel PIT separately?
> >>>>
> >>>> It was considered no longer relevant:
> >>>> http://thread.gmane.org/gmane.comp.emulators.kvm.devel/85393
> >>>>
> >>> Hmm, may be we should think about this some more. If in the (not so far)
> >>> future we want to drop pit emulation from the kernel we may want to support
> >>> -no-kvm-pit to allow migration from old kernels to new one.
> >>
> >> That's not an issue. Both device models are compatible, and you can
> >> migrate between kernel_irqchip=on/off theses days with QEMU.
> >>
> > Cool. Including PIT lost tick compensation?
> 
> The feature is not yet available for the userspace PIT (we only support
> tick compensation for the userspace RTC - IIRC). Requires better IRQ
> injection feedback, you likely remember. ;)
> 
Unfortunately I do, no matter how hard I try to forget :)

> Also, I wonder if the kernel exports all tick-compensation related
> states for save/restore. Need to check again...
> 
Looks like it does not.

--
			Gleb.