Re: [Xen-devel] modifying drivers

Pasi Kärkkäinen
Sat, 20 Feb 2010 11:58:37 -0000

On Fri, Feb 19, 2010 at 06:15:56PM -0800, Ritu kaur
wrote:
>    Hi Jeremy,
>
>    Thanks for clarification, however, what I dont understand is this(I have
>    read documents and looked into driver code). Both netfront and netback
>    registers with xenbus and monitors "vif" interface. >From
netback point of
>    view I clearly understand its communication and other stuff as I see
>    vif<domid>:<intf-id> being created in dom0. However, when I
look into
>    domU, I do not see any vif interface created(looked with ifconfig,
>    ifconfig -a commands) is it hidden from the user? In domU, I just see
>    "eth*" interfaces created. how does eth* interfaces interact
with
>    netfront? I looked under lib/modules/linux*/... for any pseudo drivers
>    which might interact with eth*, didn''t get any answers. I am
completely
>    confused. By the way I am using Debian etch 4.0 as a domU.
>

In a PV guest, xennet (frontend) driver registers the network interface as ethX.
eth0 in the PV guest corresponds to vifX.0 backend in dom0.
eth1 in the PV guest corresponds to vifX.1 backend in dom0.
(X is the domain id of the guest).

Hopefully that clears it up.

-- Pasi

>    Jeremy/Ian, have any inputs on ioctl support?
>
>    Thanks
>
>    On Fri, Feb 19, 2010 at 4:22 PM, Jeremy Fitzhardinge
<[1]jeremy@goop.org>
>    wrote:
>
>      On 02/19/2010 02:30 PM, Ritu kaur wrote:
>
>        Thanks for the clarification. In our team meeting we decided to drop
>        netback changes to support exclusive access and go with xe command
>        line or xencenter way to do it(We are using Citrix Xenserver). Had
>        couple of follow-up questions related to Xen.
>
>        1.Is it correct that netfront driver(or any *front driver) has to be
>        explicitly integrated or compiled in the guest OS? the reason I ask
>        this is,
>
>      An HVM domain can be completely unmodified, but it will be using
>      emulated hardware devices with its normal drivers.
>
>        a. In the documents I have read, it mentions guest OS can run
without
>        any modification, however, if above is true we have to make sure
guest
>        OS we use are compiled with the relevant *front drivers.
>
>      An HVM domain can use PV drivers to optimise its IO path by bypassing
>      the emulated devices and talking directly to the backends.  PV domains
>      always use PV drivers (but they''ve already been modified).
>
>        b. we had done some changes to netback and netfront(as mentioned in
>        the previous email), when compiling kernel for dom0 it includes both
>        netfront and netback and assumed via some mechanism this netfront
>        driver would be integrated/installed into guest domains when they
are
>        installed.
>
>      No.  A dom0 kernel doesn''t have much use for frontends.
They''re usually
>      present because a given kernel can run in either the dom0 or domU
roles.
>
>        2. Any front or back driver communication is via xenbus only?
>
>      Xenbus is used to pass small amounts of control/status/config
>      information between front and backends.  Bulk data transfer is usually
>      handled with shared pages containing ring buffers, and event channels
>      for event signalling.
>
>        3. Supporting ioctl calls. Our driver has ioctl support to
read/write
>        hardware registers and one solution was to use pci passthrough
>        mechanism, however, it binds the NIC to a specific domU and we do
not
>        want that. We would like to have multiple users access to hw
>        registers(mainly stats and other stuff) via guest domains and be
able
>        to access them simultaneously. For this, we decided to go with the
>        mechanism of shared memory/event channel similar to front and back
>        drivers.  Can you please provide some inputs on this?
>
>      It''s hard to make any suggestions without knowing what your
hardware is
>      or what the use-cases are for these ioctls.  Are you saying that you
>      want to give multiple domUs direct unrestricted (read only?) access to
>      the same set of registers?  What kind of stats?  Do guests need to
read
>      them at a very high rate, or could they fetch accumulated results at a
>      lower rate?
>
>         J
>
> References
>
>    Visible links
>    1. mailto:jeremy@goop.org

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