Re: [PATCH v3] enable x2APIC without interrupt remapping under KVM

Gleb Natapov
2009-06-30T19:03:07Z

On Tue, Jun 30, 2009 at 09:44:54AM -0700, Eric W. Biederman wrote:
> Gleb Natapov <gleb@redhat.com> writes:
>
> > On Tue, Jun 30, 2009 at 02:24:05AM -0700, Eric W. Biederman wrote:
> >> Gleb Natapov <gleb@redhat.com> writes:
> >>
> >> > KVM would like to provide x2APIC interface to a guest without emulating
> >> > interrupt remapping device. The reason KVM prefers guest to use x2APIC
> >> > is that x2APIC interface is better virtualizable and provides better
> >> > performance than mmio xAPIC interface:
> >> >
> >> > - msr exits are faster than mmio (no page table walk, emulation)
> >> > - no need to read back ICR to look at the busy bit
> >> > - one 64 bit ICR write instead of two 32 bit writes
> >> > - shared code with the Hyper-V paravirt interface
> >> >
> >> > Included patch changes x2APIC enabling logic to enable it even if IR
> >> > initialization failed, but kernel runs under KVM and no apic id is
> >> > greater than 255 (if there is one spec requires BIOS to move to x2apic
> >> > mode before starting an OS).
> >>
> >>
> >> How common is hotplug hardware in kvm? In particular hotplug cpus?
> >>
> > It works for Linux guests.
>
> >
> >> To support that seriously you need interrupt remapping.
> >>
> > Can you explain why?
>
> Because ioapics don't fully function according to spec,
> and the interrupt code on the hotplug path is a horrible
> terrible broken hack for ioapics.
>
Considering that interrupt remapping is fairly new feature
are you saying that hotplug (pci and cpu) on x86 is a horrible
hack on Linux?

> It is better than nothing but it certainly is not something
> I would expect to work all of the time.
>
Because of horrible code or non-complaint ioapic implementation out
there? If later this is not a big issue for KVM.

> Interrupt remapping is the one case where we have hardware
> that works according to spec and that works reasonably well.
>
I am sure when there was only one ioapic implementation in existence
it worked according to a spec (and if not spec was changed :)) Give
interrupt remapping some time and than we will see how above statement
holds.

--
Gleb.