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

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

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).

Tamas