Re: [Xen-devel] RFC: Automatically making a PCI device assignable in the config file

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 07/08/2013 08:23 PM, Konrad Rzeszutek Wilk wrote:
> On Fri, Jul 05, 2013 at 02:52:08PM +0100, George Dunlap wrote:
> > On 05/07/13 14:48, Andrew Cooper wrote:
> > > On 05/07/13 14:45, George Dunlap wrote:
> > > > On 05/07/13 14:39, Andrew Cooper wrote:
> > > > > On 05/07/13 12:01, George Dunlap wrote:
> > > > > > I've been doing some work to try to make driver domains easier to set
> > > > > > up and use.  At the moment, in order to pass a device through to a
> > > > > > guest, you first need to assign it to pciback.  This involves doing
> > > > > > one of three things:
> > > > > > * Running xl pci-assignable-add for the device
> > > > > > * Specifying the device to be grabbed on the dom0 Linux command-line
> > > > > > * Doing some hackery in /etc/modules.d
> > > > > >
> > > > > > None of these are very satisfying.  What I think would be better is if
> > > > > > there was a way to specify in the guest config file, "If device X is
> > > > > > not assignable, try to make it assignable".  That way you can have a
> > > > > > driver domain grab the appropriate device just by running "xl create
> > > > > > domnet"; and once we have the xendomains script up and running with
> > > > > > xl, you can simply configure your domnet appropriately, and then put
> > > > > > it in /etc/xen/auto, to be started automatically on boot.
> > > > > >
> > > > > > My initial idea was to add a parameter to the pci argument in the
> > > > > > config file; for example:
> > > > > >
> > > > > > pci = ['08:04.1,permissive=1,seize=1']
> > > > > >
> > > > > > The 'seize=1' would indicate that if bdf 08:04.1 is not already
> > > > > > assignable, that xl should try to make is assignable.
> > > > > >
> > > > > > The problem here is that this would need to be parsed by
> > > > > > xlu_pci_parse_bdf(), which only takes an argumen tof type
> > > > > > libxl_device_pci.
> > > > > >
> > > > > > Now it seems to me that the right place to do this "seizing" is in xl,
> > > > > > not inside libxl -- the functions for doing assignment exist already,
> > > > > > and are simple and straightforward.  But doing it in xl, but as a
> > > > > > parameter of the "pci" setting, means changing xlu_pci_parse_bdf() to
> > > > > > pass something else back, which begins to get awkward.
> > > > > >
> > > > > > So it seems to me we have a couple of options:
> > > > > > 1. Create a new argument, "pci_seize" or something like that, which
> > > > > > would be processed separately from pci
> > > > > > 2. Change xlu_pci_parse_bdf to take a pointer to an extra struct, for
> > > > > > arguments directed at xl rather than libxl
> > > > > > 3. Add "seize" to libxl_device_pci, but have it only used by xl
> > > > > > 4. Add "seize" to libxl_device_pci, and have libxl do the seizing.
> > > > > >
> > > > > > Any preference -- or any other ideas?
> > > > > >
> > > > > >    -George
> > > > > How about a setting in xl.conf of "auto-seize pci devices" ?  That way
> > > > > the seizing is entirely part of xl
> > > > Auto-seizing is fairly dangerous; you could easily accidentally yank
> > > > out the ethernet card, or even the disk that dom0 is using.  I really
> > > > think it should have to be enabled on a device-by-device basis.
> > > >
> > > > I suppose another option would be to be able to set, in xl.conf, a
> > > > list of auto-seizeable devices.  I don't really like that option as
> > > > well, though.  I'd rather be able to keep all the configuration in one
> > > > place.
> > > >
> > > >   -George
> > > Or a slight less extreme version.
> > >
> > > If xl sees that it would need seize a device, it could ask "You are
> > > trying to create a domain with device $FOO.  Would you like to seize it
> > >from dom0 ?"
> >
> > That won't work for driver domains, as we want it all to happen
> > automatically when the host is booting. :-)
> The high-level goal is that we want to put the network devices with a
> network backend and storage devices with storage backend. Ignorning
> that for network devices you might want seperate backends for each
> device (say one backend for Wireless, one for Ethernet, etc).
>
> Perhaps the logic ought to do grouping - so you say:
>   a) "backends:all-network" (which would created one backend with all of the
>     wireless, ethernet, etc PCI devices), or
>   b) "backends:all-network,seperate-storage", which  create one backend with
>    all of the wireless, ethernet in one backend; and one backend domain for each
>    storage device?
>
> Naturally the user gets to chose which grouping they would like?
We seem to be talking about different things.  You seem to be talking 
about automatically starting some pre-made VMs and assigning devices and 
backends to them?  But I'm not really sure.
I was assuming that the user was going to be installing and configuring 
their own driver domains.  The user already has to specify 
"pci=['$BDF']" in their config file to get specific devices passed 
through -- this would just be making it easy to have the device assigned 
to pciback as well.
I suspect that a lot of people will want to have one network card 
assigned to domain 0 as a "management network", and only have other 
devices assigned to driver domains.  I think that having one device per 
domain is probably the best recommendation; although we obviously want 
to support someone who wants a single "manage all the devices" domain, 
we should assume that people are going to have one device per driver domain.
 -George

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