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

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 07/09/2013 03:25 PM, Konrad Rzeszutek Wilk wrote:
> On Tue, Jul 09, 2013 at 01:52:38PM +0100, George Dunlap wrote:
> > 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 $FOO.  Would you like to seize it
> > > > >from dom0 ?"
> > > >
> > > > That won't work for driver domains, as we want it all to happen
> > > > automatically when the host is booting. :-)
> > > The high-level goal is that we want to put the network devices with a
> > > network backend and storage devices with storage backend. Ignorning
> > > that for network devices you might want seperate backends for each
> > > device (say one backend for Wireless, one for Ethernet, etc).
> > >
> > > Perhaps the logic ought to do grouping - so you say:
> > >   a) "backends:all-network" (which would created one backend with all of the
> > >     wireless, ethernet, etc PCI devices), or
> > >   b) "backends:all-network,seperate-storage", which  create one backend with
> > >    all of the wireless, ethernet in one backend; and one backend domain for each
> > >    storage device?
> > >
> > > Naturally the user gets to chose which grouping they would like?
> > We seem to be talking about different things.  You seem to be
> > talking about automatically starting some pre-made VMs and assigning
> > devices and backends to them?  But I'm not really sure.
> I am trying to look at it from a high perspective to see whether we can
> make this automated for 99% of people out of the box. Hence the
> idea of grouping. And yes to '..assigning devices and backends to them'.
> > I was assuming that the user was going to be installing and
> > configuring their own driver domains.  The user already has to
> > specify "pci=['$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 think the technical bits what libxl is doing and yanking devices
> around is driven either by the admin or a policy. If the policy
> is this idea of grouping (that is a terrible name now that I think
> of it), then perhaps we should think how to make that work and then
> the details (such as this automatic yanking of devices to pci-back)
> can be filled in.
>
>
> > 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.
> I don't know. My feeble idea was that we would have at minimum _two_
> guests on bootup. One is a control one that has no devices - but is
> the one that launches the guests.
>
> Then there is the dom1 which would have all (or some) of the storage
> and network devices plugged in along with the backends. Then a dom2
> which would be the old-style-dom0 - so it would have the graphic card
> and the rest of the PCI devices.
>
> In other words, when I boot I would have two tiny domains launch
> right before "old-style-dom0" is started. But I am getting in specifics
> here.
>
> Perhaps you could explain to me how you envisioned how the device
> driver domains idea would work? How would you want it to work on your
> laptop?
>
> Or are we right now just thinking of the small pieces of making the
> code be able to yank the devices around and assign them?
I was thinking for now just making the "manually configure it" case 
easier.  I decided to switch one of my test boxen to using a network 
driver domain by default, and although the core is there, there are a 
bunch of things that are unnecessarily crufty.
I do agree that long term it would be nice to make it easy to make 
driver domains the default, but that's not what I had in mind for this 
conversation. :-)
The hard part for making it really automated, it seems to me, comes from 
two things.  O
One, you have to make sure your driver domain has the appropriate 
hardware drivers for your system as well.  We don't want to be in the 
business of maintaining a distro; most people will probably want the 
driver domain to be from the same distro they're using for dom0, which 
means that setting up such a domain will need to be done differently on 
a distro-by-distro basis.
Two, you have the configuration problem.  In Debian, for instance, if 
you wanted to switch a device from being owned by dom0 to being in a 
driver domain, you'd have to:
* Copy over the udev rules recognizing the mac address, so it got the 
same ethN
* copy over the eth and bridge info from dom0's /etc/network/interfaces 
into the guest /etc/network/interfaces
I'm not sure exactly what you have to do in Fedora, but I bet it's 
something similar.
It might be nice to work with distros to make the process of making 
driver domains / stub domains easier, and to make it easy to configure 
driver domain networking options from the distro's network scripts; but 
that's kind of another level of functionality.
I think first things first: make manually-set-up driver domains actually 
easy to use.
 -George

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