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

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

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 thought I heard someone say that many devices will only use RMRRs for
compatibility with older OSes or during boot; in which case, there may
be devices which you can safely assign to newer OSes / hot-plug after
the guest has booted even without reserving the RMRR.  If such devices
exist, then the admin should be able to assign those, shouldn't they?

Making it "rmrr=force" by default, but allowing an admin to specify
"rmrr=try", makes sense to me.  It does introduce an extra layer of
complication, so I wouldn't push for it; but if Kevin / Intel wants to
do the work, I think it's a good thing.

 -George

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