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

[Sean Christopherson <seanjc@google.com>]

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
an immutable topic branch/tag.

My thought is to have you carry the below in the s390 series though, as I don't
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.