[Qemu-devel] Re: [RFC] Moving the kvm ioapic, pic, and pit back to userspace

[David S. Ahern]

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