On Monday 25 May 2009 14:38:11 Gleb Natapov wrote: > On Mon, May 25, 2009 at 02:30:05PM +0800, Sheng Yang wrote: > > On Monday 25 May 2009 14:13:23 Gleb Natapov wrote: > > > On Mon, May 25, 2009 at 02:08:26PM +0800, Sheng Yang wrote: > > > > On Friday 22 May 2009 01:37:53 Gleb Natapov wrote: > > > > > This is implementation of x2apic for KVM that I wrote a while ago. > > > > > Unfortunately there is no guest that can take advantage of it since > > > > > Linux doesn't (yet?) use x2apic if interrupt remapping is not > > > > > enabled and I don't feel like implement interrupt remapping device > > > > > :) > > > > > > > > > > Re-based to latest kvm tree for your viewing pleasure and feedback. > > > > > > > > Yeah... x2apic is for interrupt remapping, and interrupt remapping is > > > > for VT-d engine. So if we don't want to virtualize VT-d engine and > > > > interrupt remapping, x2apic is useless for the guest... And VT-d > > > > engine(and related things) virtualization is far from our scope > > > > now... > > > > > > Can you explain why "x2apic is for interrupt remapping"? I can > > > understand why interrupt remapping is needed to use x2apic in certain > > > circumstances (apic ids > 256). x2apic has better virtualizable > > > interface and that is why we want to emulate it. Can you name one > > > technical reason why it can't be done in a context of KVM? > > > > As you know, x2apic is for apic id > 256. So, at least on the real > > machine, if there is no interrupt remapping, x2apic almost can't take > > much advantage, because IOAPIC and MSI/MSI-X delivery still using 8bit > > apic id, so any external interrupt can't benefit from it(though IPI can > > benefit some, and maybe some gain from MSR rather than MMIO). And this is > > the main purpose for x2apic, that's why Linux kernel limited x2apic using > > with interrupt remapping. > > > > I am not sure the things looks like in the context of KVM. I think mostly > > even you virtualize it, it can't replace lapic, for guest won't use it(I > > am not sure about the Windows). Can you explain more detail? > > KVM will expose x2apic, but will use apic ids < 256. Linux has to be > changed to use x2apic if it runs as a guest. Windows, when running as a > guest, partially paravirtualize lapic by using MSRs to access frequently > used registers. The MSR interface Windows uses is very similar to x2apic > by the way and KVM will support it too. > OK, you are totally talking about PV. For PV, I think let host kernel accept the modification is more important here. (And for PV, using hypercall seems more directly). -- regards Yang, Sheng -- 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
