Re: [PATCH v4 09/16] KVM: Introduce KVM_CAP_NOWAIT_ON_FAULT without implementation

[Sean Christopherson <seanjc@google.com>]

On Thu, Jun 15, 2023, Wei W Wang wrote:
> On Thursday, June 15, 2023 5:21 AM, Sean Christopherson wrote:
> > Ah, crud.  The above highlights something I missed in v3.  The memslot NOWAIT
> > flag isn't tied to FOLL_NOWAIT, it's really truly a "fast-only" flag.  And even
> > more confusingly, KVM does set FOLL_NOWAIT, but for the async #PF case,
> > which will get even more confusing if/when KVM uses FOLL_NOWAIT internally.
> > 
> > Drat.  I really like the NOWAIT name, but unfortunately it doesn't do what as the
> > name says.
> > 
> > I still don't love "fast-only" as that bleeds kernel internals to userspace.
> > Anyone have ideas?  Maybe something about not installing new mappings?
> 
> Yes, "NOWAIT" sounds a bit confusing here. If this is a patch applied to userfaultfd
> to solve the "wait" issue on queuing/handling faults, then it would make sense.
> But this is a KVM specific solution, which is not directly related to userfaultfd, and
> it's not related to FOLL_NOWAIT. There seems nothing to wait in the KVM context
> here.
> 
> Why not just name the cap as what it does (i.e. something to indicate the cap of
> having the fault exited to userspace to handle), e.g. KVM_CAP_EXIT_ON_FAULT
> or KVM_CAP_USERSPACE_FAULT.

Because that's even further away from the truth when accounting for the fact that
the flag controls behavior when handling are *guest* faults.  The memslot flag
doesn't cause KVM to exit on every guest fault.  And USERSPACE_FAULT is far too
vague; KVM constantly faults in userspace mappings, the flag needs to communicate
that KVM *won't* do that for guest accesses.

Something like KVM_MEM_NO_USERFAULT_ON_GUEST_ACCESS?  Ridiculously verbose, but
I think it captures the KVM behavior, and "guest access" instead of "guest fault"
gives KVM some wiggle room, e.g. the name won't become stale if we figure out a
way to apply the behavior to KVM emulation of guest accesses in the future.