Re: [PATCH v5] enable x2APIC without interrupt remapping under KVM

Gleb Natapov
2009-07-04T09:56:11Z

On Sat, Jul 04, 2009 at 02:35:30AM -0700, Eric W. Biederman wrote:
> Ingo Molnar <mingo@elte.hu> writes:
>
> > * Suresh Siddha <suresh.b.siddha@intel.com> wrote:
> >
> >> On Wed, 2009-07-01 at 06:30 -0700, Gleb Natapov wrote:
> >> > 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).
> >> >
> >> > Signed-off-by: Gleb Natapov <gleb@redhat.com>
> >>
> >> Acked-by: Suresh Siddha <suresh.b.siddha@intel.com>
> >
> > Now, since this affects core x86 APIC code non-trivially so should
> > submitted to and go via the x86 tree. (Can prepare a special branch
> > with just this change if KVM tree wants/needs to pull it before
> > v2.6.32.)
>
> Please don't separate the x2apic code from the dmar code for this
> reason.
>
> Supporting hotplug cpus with ioapics is torture.
>
What is the connection between this patch and cpu hotplug?

--
Gleb.