On Tue, Jun 30, 2009 at 09:44:54AM -0700, Eric W. Biederman wrote: > 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. > 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. -- 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
