Re: [PATCH v6 26/29] KVM: Optimize gfn lookup in kvm_zap_gfn_range()

[Maciej S. Szmigiero]

On 01.12.2021 17:36, Sean Christopherson wrote:
> On Wed, Dec 01, 2021, Maciej S. Szmigiero wrote:
> > On 01.12.2021 04:41, Sean Christopherson wrote:
> > > On Tue, Nov 30, 2021, Maciej S. Szmigiero wrote:
> > > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > > > index 41efe53cf150..6fce6eb797a7 100644
> > > > --- a/include/linux/kvm_host.h
> > > > +++ b/include/linux/kvm_host.h
> > > > @@ -848,6 +848,105 @@ struct kvm_memory_slot *id_to_memslot(struct kvm_memslots *slots, int id)
> > > >    	return NULL;
> > > >    }
> > > > +/* Iterator used for walking memslots that overlap a gfn range. */
> > > > +struct kvm_memslot_iter {
> > > > +	struct kvm_memslots *slots;
> > > > +	gfn_t end;
> > > > +	struct rb_node *node;
> > > > +};
> > >
> > > ...
> > >
> > > > +static inline struct kvm_memory_slot *kvm_memslot_iter_slot(struct kvm_memslot_iter *iter)
> > > > +{
> > > > +	return container_of(iter->node, struct kvm_memory_slot, gfn_node[iter->slots->node_idx]);
> > >
> > > Having to use a helper in callers of kvm_for_each_memslot_in_gfn_range() is a bit
> > > ugly, any reason not to grab @slot as well?  Then the callers just do iter.slot,
> > > which IMO is much more readable.
> >
> > "slot" can be easily calculated from "node" together with either "slots" or
> > "node_idx" (the code above just adjusts a pointer) so storing it in the
> > iterator makes little sense if the later are already stored there.
>
> I don't want the callers to have to calculate the slot.  It's mostly syntatic
> sugar, but I really do think it improves readability.  And since the first thing
> every caller will do is retrieve the slot, I see no benefit in forcing the caller
> to do the work.
>
> E.g. in the simple kvm_check_memslot_overlap() usage, iter.slot->id is intuitive
> and easy to parse, whereas kvm_memslot_iter_slot(&iter)->id is slightly more
> difficult to parse and raises questions about why a function call is necessary
> and what the function might be doing.

Personally, I don't think it's that much less readable, but I will change
the code to store "slots" instead (as you wish) since it's the last remaining
change - other than Paolo's call whether we should keep or drop the
kvm_arch_flush_shadow_memslot()-related patch 25.

Thanks,
Maciej