Anthony Liguori <anthony <at> codemonkey.ws> writes: > > Kapadia, Vivek wrote: > > I came across this thread looking for an efficient event channel > > mechanism between two guests (running on different cpu cores). > > > > While I can use available emulated IO mechanism (guest1->host kernel > > driver->Qemu1->Qemu2) in conjunction with interrupt mechanism (Qemu2 > > ->host kernel driver->guest2) in KVM, this involves > > several context switches. Xen handles notifications in hypervisor via > > hypercall and hence is likely more efficient. > > > > They almost certainly aren't more efficient. > > An event channel notification involves a hypercall to the hypervisor. > When using VT, the performance difference between a vmcall exit vs. a > pio exit is quite small (especially compared to the overhead of the > exit). We're talking in the order of nanoseconds compared to > microseconds. I agree. The hypercall and PIO exit won't be significantly different, but what I am referring to here is a particular case of guest to guest notification and number of context switches involved in the process. While I cannot avoid heavy weight exit either way (exit via hypercall or PIO), by handling the notification in hypervisor, I would think we can reduce the overhead due to extra the context switches. > What makes KVM particularly different from Xen is that in KVM, the PIO > operation results in a direct transition to QEMU. In Xen, typically > event channel notifications result in a bit being set in a bitmap which > then results in an interrupt injection depending on the next opportunity > the hypervisor has to schedule/run the receiving domain. This is not > deterministic and can potentially be a very long period of time. > I have guests pinned to cpu cores, and hence the notifications will be synchronous. > > > Is there a way I can perform direct notification (guest1->host kernel > > driver->guest2) in kvm? > > > > Between guests, we don't have a notification framework today. You can > use IPC from two QEMU processes and I'd expect that to perform pretty > well. I'm not sure you can get much advantages from doing things in the > kernel because you cannot avoid the heavy weight exit. > Has KVM future plans to support guest-to-guest notification framework? If not, how easy or difficult would it be today to write simple guest-to-guest notification in KVM host driver? Any hints would be helpful. Thanks. > Regards, > > Anthony Liguori > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo <at> vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > > -- 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
