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

  • To: Pasi Kärkkäinen <pasik@xxxxxx>
  • From: Abhinav Srivastava <abhinavs_iitkgp@xxxxxxxxxxx>
  • Date: Sun, 1 Jun 2008 23:51:19 +0530 (IST)
  • Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
  • Delivery-date: Sun, 01 Jun 2008 11:21:45 -0700
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Hi Pasi,

Thanks for your reply.

I wanted to know how packet is being transferred from dom0 to domU and vice 
versa. My goal is to track packets and for that I need to know in which memory 
it is residing. For example, in case of PV drivers packets are passed from dom0 
to domU by copying on the shared I/O memory between two domains. This is not 
the case in HVM domain. So my questions is how domU gets packet that is coming 
through dom0 and vice-versa. How actual network packet is being copied?

Once, I know where dom0 keeps packet for domU I can check in that memory 
location. And, once packet reaches inside domU then I guess normal OS packet 
flow is follwed.

I look forward hearing from you soon.


--- On Sun, 1/6/08, Pasi Kärkkäinen <pasik@xxxxxx> wrote:

> From: Pasi Kärkkäinen <pasik@xxxxxx>
> Subject: Re: [Xen-devel] Network Packet Flow in HVM
> To: "Abhinav Srivastava" <abhinavs_iitkgp@xxxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
> Date: Sunday, 1 June, 2008, 5:13 PM
> 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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