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Date: Mon, 07 Jun 2010 10:31:54 -0600
From: "David S. Ahern" <daahern@cisco.com>
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Subject: [Qemu-devel] Re: [RFC] Moving the kvm ioapic, pic,
	and pit back to userspace
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To: Avi Kivity <avi@redhat.com>
Cc: qemu-devel <qemu-devel@nongnu.org>, KVM list <kvm@vger.kernel.org>



On 06/07/10 09:26, Avi Kivity wrote:

> The original motivation for moving the PIC and IOAPIC into the kernel
> was performance, especially for assigned devices.  Both devices are high
> interaction since they deal with interrupts; practically after every
> interrupt there is either a PIC ioport write, or an APIC bus message,
> both signalling an EOI operation.  Moving the PIT into the kernel
> allowed us to catch up with missed timer interrupt injections, and
> speeded up guests which read the PIT counters (e.g. tickless guests).
> 
> However, modern guests running on modern qemu use MSI extensively; both
> virtio and assigned devices now have MSI support; and the planned VFIO
> only supports kernel delivery via MSI anyway; line based interrupts will
> need to be mediated by userspace.

The "modern" guest comment is a bit concerning. 2.4 kernels (e.g.,
RHEL3) use the PIT for timekeeping and will still be around for a while.
RHEL4 and RHEL5 will be around for a long time to come. Not sure how
those fit within the "modern" label, though I see my RHEL4 guest is
using the pit as a timesource.

David