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

[George Dunlap <george.dunlap@eu.citrix.com>]

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. :-)

  -George