On Wed, Apr 13, 2011 at 1:09 AM, Tommaso Cucinotta
<tommaso.cucinotta@xxxxxxxx> wrote:
> I'd like to "intercept" from the host the exact times at which an incoming
> network packet directed to a guest VM:
> a) is delivered from the host OS to the KVM process;
> b) is delivered to the "CPU thread" of the KVM process.
>
> Specifically, I don't have a clean idea of how b) happens when the CPU
> thread is doing compute-intensive activities within the VM. How is the flow
> of control of such thread asynchronously interrupted so as to hand over
> control to the proper network driver in kvm ? Any pointer to the exact
> points to look at, in the KVM code, are also very well appreciated.
If you are using userspace virtio-net (not in-kernel vhost-net), then
an incoming ("rx") packet results in the qemu-kvm iothread's select(2)
system call returning with a readable tap file descriptor:
vl.c:main_loop_wait()
(During this time the vcpu thread may still be executing guest code.)
The iothread runs the tap receive function:
net/tap.c:tap_send()
The iothread places the received packet into the rx virtqueue and
interrupts the guest:
hw/virtio-net.c:virtio_net_receive()
hw/virtio-pci.c:virtio_pci_notify()
The interrupt is injected by the KVM kernel module:
arch/x86/kvm/x86.c:kvm_arch_vm_ioctl() KVM_IRQ_LINE
There is some guest mode exiting logic here to kick the vcpu:
arch/x86/kvm/lapic.c:__apic_accept_irq()
During this whole time the vcpu may be executing guest code. Only at
the very end has the interrupt been inject and the vcpu notified.
Stefan
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