Re: [Xen-devel] (v2) Design proposal for RMRR fix

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 01/14/2015 02:32 PM, Jan Beulich wrote:
>>>> On 14.01.15 at 13:01, <george.dunlap@xxxxxxxxxxxxx> wrote:
>> On 01/14/2015 10:24 AM, Jan Beulich wrote:
>>>>>> On 14.01.15 at 10:43, <kevin.tian@xxxxxxxxx> wrote:
>>>>>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>>>>> Sent: Wednesday, January 14, 2015 5:00 PM
>>>>>
>>>>>>>> On 14.01.15 at 09:06, <kevin.tian@xxxxxxxxx> wrote:
>>>>>> Now the open is whether we want to fail domain creation for all of above
>>>>>> conflicts. user may choose to bear with conflicts at his own disposal, or
>>>>>> libxl doesn't want to fail conflicts as preparation for future
>>>>>> hotplug/migration.
>>>>>> One possible option is to add a per-region flag to specify whether 
>>>>>> treating
>>>>>> relevant conflict as an error, when libxl composes the list to domain
>>>>>> builder.
>>>>>> and this information will be saved in a user space database accessible to
>>>>>> all components and also waterfall to Xen hypervisor when libxl requests
>>>>>> actual device assignment.
>>>>>
>>>>> That's certainly a possibility, albeit saying (in the guest config) that
>>>>> a region to be reserved only when possible is about the same as
>>>>> not stating that region. If at all, I'd see the rmrr-host value be a
>>>>> tristate (don't, try, and force) to that effect.
>>>>>
>>>>
>>>> how about something like below with bi-state?
>>>>
>>>> for statically assigned device:
>>>>    pci = [ "00:02.0, 0/1" ]
>>>> where '0/1' represents try/force (or use 'try/force', or have a meaningful 
>>>> attribute like rmrr_check=try/force?)
>>>
>>> As said many times before, for statically assigned devices such a flag
>>> makes no sense.
>>>
>>>> for other usages like hotplug/migration:
>>>>    reserved_regions = [ 'host, 0/1', 'start, end, 0/1', 'start, end, 0/1', 
>>>> ...]
>>>> If 'host' is specified, it implies rmrr_host, besides user can specific 
>>>> explicit ranges according to his detail requirement.
>>>
>>> For host the flag makes sense, but for the explicitly specified regions
>>> - as said before - I don't think it does.
>>
>> You don't think there are any circumstances where an admin should be
>> allowed to "shoot himself in the foot" by assigning a device which he
>> knows the RMRRs conflict -- perhaps because he "knows" that the RMRRs
>> won't actually be used?
> 
> I did advocate for allowing this, and continue to do so. But I think
> the necessary override for this would apply at assignment time,
> not when punching the holes (i.e. would need to be a different
> setting).

But essentially what you're saying then is that for such devices, you
should not be able to statically assign them; you are only allowed to
hotplug them.

If you want to statically assign such a device, then libxl *should* try
to make the RMRR if possible, but shouldn't fail if it can't; and, it
needs to tell Xen not to fail the assignment when setting up the domain.

For that purpose, adding "rmrr=try" to the pci config spec makes the
most sense, doesn't it?

Or am I missing something?

 -George

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