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

Ingo Molnar
2009-07-03T08:29:13Z

* 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.)

Ingo