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

[Jan Beulich <jbeulich@xxxxxxxx>]

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).

Jan