Re: (v2) Design proposal for RMRR fix

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

On 01/14/2015 12:14 PM, Ian Campbell wrote:
> On Wed, 2015-01-14 at 06:52 +0000, Tian, Kevin wrote:
>>> From: Jan Beulich [mailto:JBeulich@suse.com]
>>> Sent: Wednesday, January 14, 2015 12:06 AM
>>>
>>>>>> On 13.01.15 at 17:00, <george.dunlap@eu.citrix.com> wrote:
>>>> Another option I was thinking about: Before assigning a device to a
>>>> guest, you have to unplug the device and assign it to pci-back (e.g.,
>>>> with xl pci-assignable-add).  In addition to something like rmmr=host,
>>>> we could add rmrr=assignable, which would add all of the RMRRs of all
>>>> devices currently listed as "assignable".  The idea would then be that
>>>> you first make all your devices assignable, then just start your guests,
>>>> and everything you've made assignable will be able to be assigned.
>>>
>>> Nice idea indeed, but I'm not sure about its practicability: It may
>>> not be desirable to make all devices eventually to be handed to a
>>> guest prior to starting any of the guests it may get handed to. In
>>> particular there may be reasons why the host needs the device
>>> while (or until after) creating the guests.
>>>
>>
>> and I'm not sure whether there's enough knowledge to judge whether 
>> a device is assignable since potential conflicts may be detected only
>> when the guest is launched.
> 
> I don't think George was intending to imply otherwise, assignable here
> just means "bound to xen-pciback", there may be other reasons why the
> device cannot be assigned in practice when you come to actually use it,
> i.e. RMRR conflicts which may only be discovered when a guest is started
> would be one such practical reason.

Yes -- xl has a concept called "pci-assignable".  Before you can add a
device to a guest, you have to call "xl pci-assignable-add [device
spec]".  You can also run "xl pci-assignable-list" to see which devices
are currently assignable.

Normally this is true even for statically-assigned devices: If you add
pci= [ "$bdi" ] to a config file, and $bdi hasn't been made assignable,
then the pci-attach in domain creation will fail and the domain will be
destroyed.  You can make the domain builder do this automatically with
the "seize=1" parameter; i.e., pci = [ "$bdf,seize=1" ].

My suggestion was that in addition to specifying the particular ranges,
and specifying rmrr=host, we could also specify "rmrr=assignable", which
would cause the domain builder to internally run
libxl_pci_assignable_list() and find the RMRRs for all devices on the list.

But as Ian says, that's a "nice to have", not a requirement.

 -George