Re: [Xen-devel] [PATCH v8 13/15] xen: make grant resource limits per domain

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.09.17 at 10:27, <jgross@xxxxxxxx> wrote:
> On 22/09/17 09:53, Jan Beulich wrote:
>>>>> On 22.09.17 at 08:19, <jgross@xxxxxxxx> wrote:
>>> On 21/09/17 13:48, Jan Beulich wrote:
>>>>>>> On 21.09.17 at 13:39, <jgross@xxxxxxxx> wrote:
>>>>> On 21/09/17 13:31, Jan Beulich wrote:
>>>>>>>>> On 21.09.17 at 09:53, <jgross@xxxxxxxx> wrote:
>>>>>>> On 21/09/17 08:15, Jan Beulich wrote:
>>>>>>>>>>> On 21.09.17 at 06:35, <jgross@xxxxxxxx> wrote:
>>>>>>>>> On 20/09/17 17:35, Jan Beulich wrote:
>>>>>>>>>>>>> On 20.09.17 at 14:44, <jgross@xxxxxxxx> wrote:
>>>>>>>>>>> On 20/09/17 13:48, Jan Beulich wrote:
>>>>>>>>>>>>>>> On 20.09.17 at 13:10, <jgross@xxxxxxxx> wrote:
>>>>>>>>>>>>> I thought about a cap and TBH I'm not sure which would be sane to
>>>>>>>>>>>>> apply. The global limits seem wrong, especially looking at patch 
>>>>>>>>>>>>> 14:
>>>>>>>>>>>>> those limits will be for dom0 only then. And dom0 won't need many
>>>>>>>>>>>>> grant frames in the normal case...
>>>>>>>>>>>>
>>>>>>>>>>>> I've been thinking about this Dom0 aspect too over lunch. What
>>>>>>>>>>>> about allowing the hardware domain to set its limit (only upwards
>>>>>>>>>>>> of course) in setup_table(), without any upper bound enforced?
>>>>>>>>>>>> This would free up the globals to be used as system wide limits
>>>>>>>>>>>> again.
>>>>>>>>>>>
>>>>>>>>>>> This would be possible, of course.
>>>>>>>>>>>
>>>>>>>>>>> The question is whether the need to re-allocate the frame pointer 
>>>>>>>>>>> arrays
>>>>>>>>>>> is it worth.
>>>>>>>>>>
>>>>>>>>>> Input by others would be helpful...
>>>>>>>>>
>>>>>>>>> I think I'll go with additional cap boot parameters, so I don't think
>>>>>>>>> we need dom0 to modify its own limits.
>>>>>>>>
>>>>>>>> So are we in agreement then that no new command line options
>>>>>>>> are needed, and that hence the cap will be applicable to all
>>>>>>>> domains (with Dom0 simply not having any other limit enforced
>>>>>>>> on it)?
>>>>>>>
>>>>>>> Hmm, I meant this to be the other way round: having distinct parameters
>>>>>>> for dom0 and the cap.
>>>>>>>
>>>>>>> In case you like it much better to merge them I won't argue over it.
>>>>>>
>>>>>> Well, late yesterday evening it occurred to me that it would
>>>>>> only be consistent to apply the same cap to all domains. That's
>>>>>> in particular to not penalize a non-Dom0 hardware domain in
>>>>>> comparison with Dom0 itself.
>>>>>
>>>>> That's correct.
>>>>>
>>>>> OTOH e.g. a cap of lets say 1024 grant frames but Dom0 configured to
>>>>> 4 only (why would it need more?) would make sense: the grant frame array
>>>>> for Dom0 would need 32 bytes only instead of the 8kB for the 1024 frames
>>>>> if the cap would be the configuration value for Dom0.
>>>>
>>>> May I suggest that for now we use the simpler variant without
>>>> extra Dom0 command line options, and later (post 4.10), if you or
>>>> anyone else really feels like it, Dom0 specific options be introduced?
>>>
>>> While applying these changes to my series I realized this might be a bad
>>> choice for ARM: the dom0 grant table is here limited to about 100 pages.
>>> If there is some need to have a domU with more grant frames the system
>>> wouldn't be able to boot as the high cap would be used for the dom0
>>> grant frame number.
>> 
>> Why can't ARM code lower the Dom0 values without lowering the
>> caps?
> 
> So either we let control the max_grant_frames value the cap _and_ the
> dom0 value or not. We could handle this differently on ARM, of course,
> but this would mean that the dom0 value on ARM wouldn't be adjustable
> other than as a compile time option.

Why? If the specified value is lower than the about 100 pages
allow for, it could still take effect.

> Or we could do that on x86, too.

Not without an actual need to, I would say.

> For setting a compile time value of dom0 I'd go with a rather low value
> like INITIAL_NR_GRANT_FRAMES.
> 
> In the end having a sub-option for dom0 isn't that complicated, IMO.

That's true, but the inflation of command line options is by itself
worrying to me.

Jan

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