Re: [PATCH v1 02/13] KVM: s390: fake memslots for ucontrol VMs

Claudio Imbrenda
2025-01-10T17:43:28Z

On Fri, 10 Jan 2025 09:34:49 -0800
Sean Christopherson <seanjc@google.com> wrote:

> On Fri, Jan 10, 2025, Claudio Imbrenda wrote:
> > On Fri, 10 Jan 2025 08:22:12 -0800
> > Sean Christopherson <seanjc@google.com> wrote:
> > > AFAIK, that limitation exists purely because of dirty bitmaps. IIUC, these "fake"
> > > memslots are not intended to be visible to userspace, or at the very least don't
> > > *need* to be visible to userspace.
> > >
> > > Assuming that's true, they/it can/should be KVM-internal memslots, and those
> > > should never be dirty-logged. x86 allocates metadata based on slot size, so in
> > > practice creating a mega-slot will never succeed on x86, but the only size
> > > limitation I see in s390 is on arch.mem_limit, but for ucontrol that's set to -1ull,
> > > i.e. is a non-issue.
> > >
> > > I have a series (that I need to refresh) to provide a dedicated API for creating
> > > internal memslots, and to also enforce that flags == 0 for internal memslots,
> > > i.e. to enforce that dirty logging is never enabled (see Link below). With that
> > > I mind, I can't think of any reason to disallow a 0 => TASK_SIZE memslot so long
> > > as it's KVM-defined.
> > >
> > > Using a single memslot would hopefully allow s390 to unconditionally carve out a
> > > KVM-internal memslot, i.e. not have to condition the logic on the type of VM. E.g.
> >
> > yes, I would love that
> >
> > the reason why I did not use internal memslots is that I would have
> > potentially needed *all* the memslots for ucontrol, and instead of
> > reserving, say, half of all memslots, I decided to have them
> > user-visible, which is hack I honestly don't like.
> >
> > do you think you can refresh the series before the upcoming merge
> > window?
>
> Ya, I'll refresh it today, and then I can apply it early next week and provide

excellent, thanks!

> an immutable topic branch/tag.
>
> My thought is to have you carry the below in the s390 series though, as I don't

sure

> have a way to properly test it, and I'd prefer to avoid having to do a revert on
> the off chance removing the limit doesn't work for ucontrol.

makes sense, yes