Gleb Natapov <gleb@xxxxxxxxxx> writes: > On Tue, Jun 30, 2009 at 02:24:05AM -0700, Eric W. Biederman wrote: >> Gleb Natapov <gleb@xxxxxxxxxx> 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. It is better than nothing but it certainly is not something I would expect to work all of the time. Interrupt remapping is the one case where we have hardware that works according to spec and that works reasonably well. Eric -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
