Re: Resecting "options UVM_HOTPLUG"

[Santhosh Raju <fox%netbsd.org@localhost>]

On Fri, Dec 1, 2023 at 6:42 AM Mathew Cherry G. <c%bow.st@localhost> wrote:
>
> Hi,
>
> After some discussion with Jason, and re-reading the code and its
> current state in code, I've come to the conclusion that it's in the
> interest of brevity to remove everything in sys/kern/ and sys/uvm/ which
> implement the "dynamic" part of uvm_hotplug(9).
>
> The top two reasons for this in my mind are:
>
> 1) There are no current users of the uvm_hotplug(9) KPI - balloon(9)
>    uses it, but the unplug is unstable (wasn't able to debug this) and
>    is thus disabled by default.
>
> 2) In order to do unplug without a balloon(9) like mechanism (eg: on
>    native), there's a lot more state management code needed within
>    uvm(9) - to make sure that RAM segments being unplugged have no
>    machine references (eg: pmap related, pointers from inactive page
>    tables, TLB pointers, etc. etc. )
>
>    This is a lot of per-architecture work, and there doesn't seem to be
>    much demand for this feature (at least I haven't seen any comment
>    related to this as a usecase).
>

I agree with these points and if the "dynamic" part is not being used.
I am assuming we are not completely removing the UVM_HOTPLUG option?

> I believe that the project has had three useful purposes:
>
> 1) Demonstrated NetBSD code discipline - such a core part of the kernel
>    code was developed entirely in userspace prior to integration - and
>    due to the cross-compilable toolchain - it was done on Windows by
>    fox@!
>
> 2) The re-org forced better modularisation in the relevant
>    uvm/uvm_*.[hc] - ensuring even cleaner interfaces.
>
> 3) Demonstrated how TDD can reduce the pain of software dev. See:
>    https://www.netbsd.org/gallery/presentations/santhosh/2017_AsiaBSDCon/ABC2017-P8B-uvm_hotplug-paper.pdf
>
>
> If there's no specific objection to this I'll be working with Jason to
> help make this is as painless as possible.
>

I am happy to offer any assistance needed to remove the related code
if needed since I did work on the tests part of it.

>
> Many Thanks!
> --
> MattC/(~cherry)

Regards
Santhosh