Re: [XEN PATCH v1] xen: mem_access: conditionally compile vm_event.c & monitor.c

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.01.2025 12:29, Sergiy Kibrik wrote:
> 07.01.25 10:46, Jan Beulich:
>> On 06.01.2025 19:09, Tamas K Lengyel wrote:
>>> On Mon, Jan 6, 2025 at 10:10 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>>
>>>> On 06.01.2025 15:05, Tamas K Lengyel wrote:
>>>>> On Mon, Jan 6, 2025 at 5:16 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>>>>
>>>>>> On 30.12.2024 07:30, Sergiy Kibrik wrote:
>>>>>>> From: Stefano Stabellini <stefano.stabellini@xxxxxxx>
>>>>>>>
>>>>>>> Extend coverage of CONFIG_MEM_ACCESS option and make the build of VM 
>>>>>>> events
>>>>>>> and monitoring support optional.
>>>>>>
>>>>>> Yet doesn't this end up in things becoming misleading? Don't we rather 
>>>>>> need a
>>>>>> 2nd Kconfig option, with a dependency between the two? Or alternatively a
>>>>>> rename of the existing option (to describe the higher-level feature 
>>>>>> rather
>>>>>> than the lower level one)? Tamas, I'm particularly interested in knowing 
>>>>>> your
>>>>>> view here as well.
>>>>>
>>>>> Thanks Jan, I was thinking the same thing. The dependency of these
>>>>> subsystems is mem_access -> monitor -> vm_event. If the goal here is
>>>>> to disable all three levels the ideal way would be to have separate
>>>>> kconfig options for each level. It may be a bit too fine-grained
>>>>> though on ARM since there are only two types of events for monitor
>>>>> (SMC & mem_access) and only the monitor uses the vm_event channel (no
>>>>> mem-sharing/paging on ARM). So if doing separate kconfig for each
>>>>> individual feature is an overkill I would suggest using
>>>>> CONFIG_VM_EVENT that disables all three levels, including both
>>>>> mem_access & smc monitor hooks.
>>>>
>>>> Except that "disables all three levels" doesn't work, unless the other
>>>> option(s) are promptless (and selected). I'd have expected VM_EVENT to
>>>> maybe have a "depends on MEM_ACCESS", whereas a "select MEM_ACCESS"
>>>> wouldn't make much sense as long as MEM_ACCESS can be enabled
>>>> individually (with it being unclear to me whether such a configuration
>>>> is actually useful in any way).
>>>
>>> Not sure I follow. None of these systems make sense to enable
>>> individually. Without vm_event monitor/mem_access are useless, that's
>>> why I would pick CONFIG_VM_EVENT as the option on ARM to disable all
>>> three levels if we don't want to start splitting it into multiple
>>> kconfig options (which I think may be an overkill here).
>>
>> Oh, okay, you suggest to replace MEM_ACCESS by VM_EVENT at the Kconfig
>> level. That would be fine with me, so long as it's also appropriate on
>> (in particular) x86. Then, if there was ever a 2nd use of mem-access,
>> MEM_ACCESS could be re-introduced as a standalone option.
> 
> Thanks Jan,
> would it be ok to replace MEM_ACCESS with VM_EVENT everywhere at once, 
> including in defconfigs and automation script etc? Or such changes would 
> better be done gradually, starting with changing Kconfig only?

Things need to remain in sync throughout the tree, so I don't think you
can leave out e.g. defconfigs when doing the renamed. Similarly stuff
under automation/ may need changing at the same time.

Jan