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

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

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