Re: [RFC PATCH 1/4] kconfig: allow configuration of maximum modules

[Jan Beulich <jbeulich@suse.com>]

On 01.06.2022 19:35, Julien Grall wrote:
> 
> 
> On 31/05/2022 11:53, Daniel P. Smith wrote:
>> On 5/31/22 05:25, Julien Grall wrote:
>>> Hi,
>>>
>>> On 31/05/2022 03:41, Daniel P. Smith wrote:
>>>> diff --git a/xen/arch/Kconfig b/xen/arch/Kconfig
>>>> index f16eb0df43..57b14e22c9 100644
>>>> --- a/xen/arch/Kconfig
>>>> +++ b/xen/arch/Kconfig
>>>> @@ -17,3 +17,15 @@ config NR_CPUS
>>>>          For CPU cores which support Simultaneous Multi-Threading or
>>>> similar
>>>>          technologies, this the number of logical threads which Xen will
>>>>          support.
>>>> +
>>>> +config NR_BOOTMODS
>>>> +    int "Maximum number of boot modules that a loader can pass"
>>>> +    range 1 64
>>>
>>> OOI, any reason to limit the size?
>>
>> I modelled this entirely after NR_CPUS, which applied a limit
> 
> The limit for NR_CPUS makes sense because there are scalability issues 
> after that (although 4095 seems quite high) and/or the HW impose a limit.

The 4095 is actually a software limit (due to how spinlocks are
implemented).

>> , and it
>> seemed reasonable to me at the time. I choose 64 since it was double
>> currently what Arm had set for MAX_MODULES. As such, I have no hard
>> reason for there to be a limit. It can easily be removed or adjusted to
>> whatever the reviewers feel would be appropriate.
> 
> Ok. In which case I would drop the limit beause it also prevent a users 
> to create more 64 dom0less domUs (actually a bit less because some 
> modules are used by Xen). I don't think there are a strong reason to 
> prevent that, right?

At least as per the kconfig language doc the upper bound is not
optional, so if a range is specified (which I think it should be,
to enforce the lower limit of 1) and upper bound is needed. To
address your concern with dom0less - 32768 maybe?

Jan