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Re: [Xen-devel] Network Packet Flow in HVM

On Sun, Jun 01, 2008 at 09:50:08AM +0530, Abhinav Srivastava wrote:
> Hi there,
> I am working on a project in which I have to track a packet flow from dom0 to 
> domU. I know how this is done in case of Paravirtualized (PV) guest
> domain. In case of PV's split model, dom0 (backend) and domU (frontend) share 
> I/O channel that is setup during the domU boot and it is used to copy packets 
> between two domains.
> In case of HVM domain, there is no shared memory between dom0 and domU then 
> how this packet copying is being performed?  I know QEMU-dm is being used in 
> dom0 to emulate the network interface for domU. 
> If there is no memory then how domU get packets from dom0 and vice-versa? 
> Does Xen pass actual packet from dom0 to domU which is very costly?
> It would be really helpful if you could let me know exact packet flow as it 
> would help me understand the networking subsystem. I tried finding some 
> document but there are no publicly availabe documents that describe network 
> virtualization and packet flow in case of HVM domain.
> I would really appreciate your reply and pointers.

Yes, qemu emulated virtual NIC is pretty slow for HVM domains..

Luckily you can run paravirtual _drivers_ insite HVM domU so then you can
bypass the emulation.. almost like with "full" paravirtual domU.

-- Pasi

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