Messages in this thread |  | | | From | (Eric W. Biederman) | | Date | Sat, 04 Jul 2009 07:33:39 -0700 | | Subject | Re: [PATCH v5] enable x2APIC without interrupt remapping under KVM |
Gleb Natapov <gleb@redhat.com> writes:
> 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?
When I asked if cpu hotplug was a supported and more or less common feature of kvm I was told it was.
Good cpu hotplug today means supporting interrupt remapping. (The code you are disabling for kvm).
Therefore I don't see the point of supporting one without the other. Especially if we don't have a case where on real hardware we need to split the support.
Eric
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