Re: RFC: Automatically making a PCI device assignable in the config file

[Andrew Cooper <andrew.cooper3@citrix.com>]

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 ?"

I do think that libxl is not the correct place to have any logic like this.

~Andrew