linux-kernel - Re: [PATCH 2/7] KVM: x86/MMU: Move rmap

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Date:   Tue, 13 Dec 2022 16:12:17 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vipin Sharma <vipinsh@...gle.com>
Subject: Re: [PATCH 2/7] KVM: x86/MMU: Move rmap_iterator to rmap.h

On Fri, Dec 9, 2022 at 3:04 PM David Matlack <dmatlack@...gle.com> wrote:
>
> On Tue, Dec 06, 2022 at 05:35:56PM +0000, Ben Gardon wrote:
> > In continuing to factor the rmap out of mmu.c, move the rmap_iterator
> > and associated functions and macros into rmap.(c|h).
> >
> > No functional change intended.
> >
> > Signed-off-by: Ben Gardon <bgardon@...gle.com>
> > ---
> >  arch/x86/kvm/mmu/mmu.c  | 76 -----------------------------------------
> >  arch/x86/kvm/mmu/rmap.c | 61 +++++++++++++++++++++++++++++++++
> >  arch/x86/kvm/mmu/rmap.h | 18 ++++++++++
> >  3 files changed, 79 insertions(+), 76 deletions(-)
> >
> [...]
> > diff --git a/arch/x86/kvm/mmu/rmap.h b/arch/x86/kvm/mmu/rmap.h
> > index 059765b6e066..13b265f3a95e 100644
> > --- a/arch/x86/kvm/mmu/rmap.h
> > +++ b/arch/x86/kvm/mmu/rmap.h
> > @@ -31,4 +31,22 @@ void free_pte_list_desc(struct pte_list_desc *pte_list_desc);
> >  void pte_list_remove(u64 *spte, struct kvm_rmap_head *rmap_head);
> >  unsigned int pte_list_count(struct kvm_rmap_head *rmap_head);
> >
> > +/*
> > + * Used by the following functions to iterate through the sptes linked by a
> > + * rmap.  All fields are private and not assumed to be used outside.
> > + */
> > +struct rmap_iterator {
> > +     /* private fields */
> > +     struct pte_list_desc *desc;     /* holds the sptep if not NULL */
> > +     int pos;                        /* index of the sptep */
> > +};
> > +
> > +u64 *rmap_get_first(struct kvm_rmap_head *rmap_head,
> > +                 struct rmap_iterator *iter);
> > +u64 *rmap_get_next(struct rmap_iterator *iter);
> > +
> > +#define for_each_rmap_spte(_rmap_head_, _iter_, _spte_)                      \
> > +     for (_spte_ = rmap_get_first(_rmap_head_, _iter_);              \
> > +          _spte_; _spte_ = rmap_get_next(_iter_))
> > +
>
> I always found these function names and kvm_rmap_head confusing since
> they are about iterating through the pte_list_desc data structure. The
> rmap (gfn -> list of sptes) is a specific application of the
> pte_list_desc structure, but not the only application. There's also
> parent_ptes in struct kvm_mmu_page, which is not an rmap, just a plain
> old list of ptes.
>
> While you are refactoring this code, what do you think about doing the
> following renames?
>
>   struct kvm_rmap_head  -> struct pte_list_head
>   struct rmap_iterator  -> struct pte_list_iterator
>   rmap_get_first()      -> pte_list_get_first()
>   rmap_get_next()       -> pte_list_get_next()
>   for_each_rmap_spte()  -> for_each_pte_list_entry()
>
> Then we can reserve the term "rmap" just for the actual rmap
> (slot->arch.rmap), and code that deals with sp->parent_ptes will become
> a lot more clear IMO (because it will not longer mention rmap).
>
> e.g. We go from this:
>
>   struct rmap_iterator iter;
>   u64 *sptep;
>
>   for_each_rmap_spte(&sp->parent_ptes, &iter, sptep) {
>      ...
>   }
>
> To this:
>
>   struct pte_list_iterator iter;
>   u64 *sptep;
>
>   for_each_pte_list_entry(&sp->parent_ptes, &iter, sptep) {
>      ...
>   }

I like this suggestion, and I do think it'll make things more
readable. It's going to be a huge patch to rename all the instances of
kvm_rmap_head, but it's probably worth it.