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Re: [Xen-users] Does I/O from Dom0 skip the PV backend drivers ?

To: Joe Armstrong <Joe.Armstrong@xxxxxxxxx>
Subject: Re: [Xen-users] Does I/O from Dom0 skip the PV backend drivers ?
From: Paolo Victor <paolovictor@xxxxxxxxx>
Date: Wed, 29 Apr 2009 22:17:10 -0300
Cc: xen-users@xxxxxxxxxxxxxxxxxxx
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From what I've read at [1]:
When network packets are received by the NIC, the NIC will
raise an interrupt. While the NIC’s device driver in the driver do-
main may directly access the NIC, all interrupts in the system must
first go through the hypervisor. The hypervisor receives the inter-
rupt, determines that the NIC is owned by the driver domain, and
then generates a virtual interrupt for the NIC’s device driver in the
driver domain. The device driver receives the virtual interrupt, pro-
cesses the packets received by the NIC and sends them to the Eth-
ernet bridge. The Ethernet bridge demultiplexes the packet based
on its Ethernet address and delivers it to the appropriate netback
interface. The netback driver then sends the packet to the netfront
driver in the guest domain over an I/O channel. The netfront driver
then delivers the packet to the guest operating system as if it had
come directly from the NIC. The process is basically reversed to
send packets from the guest domains.
So if when they say "processes" at "The device driver receives the virtual interrupt, processes the packets received by the NIC" they mean "receive if the destination is Dom0" then yes, it does perform I/O directly - at least on the receive path.

Actually, running the benchmark at Dom0 was how they "simulated" a direct I/O on [2] :-)

Please correct me if I'm mistaken. I hope it helps,

[1] http://www.hpl.hp.com/techreports/2009/HPL-2009-25.pdf
[2] http://www.usenix.org/events/usenix08/tech/full_papers/santos/santos_html/

On Wed, Apr 29, 2009 at 5:45 PM, Joe Armstrong <Joe.Armstrong@xxxxxxxxx> wrote:

I am trying to determine the amount of overhead (if any) that a PV guest
incurs as compared to running on bare metal.  I was thinking of running some
performance tests on both a bare metal system and then in a PV guest – but I
only have one machine.  I am trying to figure out if  booting from a xen to
non-xen kernel is needed in order to get real numbers.

So the question boils down to – When Dom0 performs I/O does it go directly
to the physical device driver or does it also route through the PV backend
drivers ?

Also, my assumption is that the xen hypervisor has no role in any I/O
operations so it being present or not should have almost no impact on I/O

Thanks for any info.


View this message in context: http://www.nabble.com/Does-I-O-from-Dom0-skip-the-PV-backend-drivers---tp23302372p23302372.html
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