On 16.05.2010, at 11:09, Avi Kivity wrote: > On 05/16/2010 12:01 PM, Alexander Graf wrote: >> >>> That's what the world looked like in 2006. >>> >>> We could change it, but there's not much point, since having the local apic in the kernel is pretty much a requirement for reasonable performance. >>> >> Well, I'm not convinced yet that's the case for PPC as well. The timer is in-cpu anyways and I don't see why IPIs should be slow with a userspace pic - if we keep the overhead low. >> > > If it's at all possible keep the mpic out. I am _not_ advocating pushing ppc's mpic into the kernel. > >> So let me think this through. With remote interrupt injection we have. >> >> * thread 1 does vcpu_run >> * thread 2 triggers KVM_INTERRUPT on fd >> * thread 2 signals thread 1 so we're sure the interrupt gets injected >> * thread 1 exits into qemu >> > > This doesn't seem necessary. The kernel can own the interrupt line, so it remembers it from the last KVM_INTERRUPT. It's not? On signals we always exit to userspace, no? > >> * thread 1 goes back into the vcpu, triggering an interrupt >> >> Without we have: >> >> * thread 1 does vcpu_run >> * thread 2 wants to trigger an an interrupt, sets the qemu internal bit >> * thread 2 signals thread 1 so we're sure the interrupt gets processed >> * thread 1 exits into qemu >> * thread 1 triggers KVM_INTERRUPT on fd >> * thread 1 goes into the vcpu >> >> So we don't really buy anything from doing the remote injection. Hrm. >> > > Not if you make interrupt injection a lightweight exit. Please elaborate. > >> What's somewhat striking me here though is - why do we need KVM_INTERRUPT when there's all those kvm_run fields? Can't we just do interrupt injection by setting run->trigger_interrupt? There's only a single "interrupt line" on the CPU anyways. That way we'd save the ioctl and get rid of the locking problem altogether. >> > > That's what x86 does. However, it's synchronous. For everyone except for the vcpu thread executing the interrupt, it's asynchronous, right? The same applies to an in-kernel pic. Alex -- 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
