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

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Message-ID: <Y5O+/1CYivRishFE@google.com>
Date:   Fri, 9 Dec 2022 15:04:31 -0800
From:   David Matlack <dmatlack@...gle.com>
To:     Ben Gardon <bgardon@...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 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) {
     ...
  }