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

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Thu, Jul 11, 2013 at 12:35 PM, David Vrabel <david.vrabel@xxxxxxxxxx> 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.

If people really object to the extra flag, I can write up a patch
without it, but I'd prefer to have something...

 -George

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