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

[David Vrabel <david.vrabel@citrix.com>]

On 12/07/13 10:36, George Dunlap wrote:
> On Thu, Jul 11, 2013 at 12:35 PM, David Vrabel <david.vrabel@citrix.com> wrote:
>> On 10/07/13 15:48, George Dunlap wrote:
>>> On 10/07/13 14:53, Ian Jackson wrote:
>>>> George Dunlap writes ("RFC: Automatically making a PCI device
>>>> assignable in the config file"):
>>>>> 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.
>>>> I think it's a design error that this isn't done automatically by
>>>> default.
>>>>
>>>> It would be nice if there was a safety check that the device isn't
>>>> currently in use by dom0, but I don't think it's essential.  We could
>>>> just put a note in the docs saying "if you specify your dom0 nic it
>>>> will go away, duh" or something.
>>>
>>> I think it's a really bad interface decision if a simple typo might
>>> result in you yanking out your disk.
>>
>> I don't think this proposal really helps with avoiding this.  I think
>> most people will end up always adding 'seize=1' because to avoid having
>> to do so means altering config files elsewhere and rebooting.
> 
> I guess what I'm worried about is the fact that we would be changing
> things that are now "safe" to things that are not safe.  At the
> moment, "xl pci-assignable-add" might yank out a system device if you
> make a typo; but it was introduced that way, so people always had to
> be careful.  But currently, "pci=[]" and "xl pci-attach" do *not*
> behave that way; you have to make the device assignable first.  So you
> don't need to be particularly careful.  Adding "seize" at least should
> flag up to people that they need to double-check.

You could see if you can determine if a PCI device provides either a
block device or a network device and then check whether the block device
is mounted or the network device has a IP address configured.

These checks should probably be handled by a script that xl calls.

David