Re: [RFC PATCH 5/6] KVM: x86/mmu: Avoid memslot lookup in rmap_add

[David Matlack <dmatlack@google.com>]

On Tue, Aug 17, 2021 at 5:03 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 13/08/21 22:35, David Matlack wrote:
> > Avoid the memslot lookup in rmap_add by passing it down from the fault
> > handling code to mmu_set_spte and then to rmap_add.
> >
> > No functional change intended.
> >
> > Signed-off-by: David Matlack <dmatlack@google.com>
>
> I think before doing this we should take another look at the aguments
> for make_spte, set_spte and mmu_set_spte.  St
>
> static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
>                          u64 *sptep, unsigned int pte_access, bool write_fault,
>                          int level, gfn_t gfn, kvm_pfn_t pfn, bool speculative,
>                          bool host_writable)
>
> static int set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
>                      u64 *sptep, unsigned int pte_access, int level,
>                      gfn_t gfn, kvm_pfn_t pfn, bool speculative,
>                      bool can_unsync, bool host_writable)
>
> int make_spte(struct kvm_vcpu *vcpu, unsigned int pte_access, int level,
>                       gfn_t gfn, kvm_pfn_t pfn, u64 old_spte, bool speculative,
>                       bool can_unsync, bool host_writable, bool ad_disabled,
>                       u64 *new_spte)
>
> In particular:
>
> - set_spte should be inlined in its two callers.  The SET_SPTE_*
> flags are overkill if both functions can just call make_spte+mmu_spte_update:
> mmu_set_spte can check *sptep == spte and return RET_PF_SPURIOUS directly,
> while SET_SPTE_NEED_REMOTE_TLB_FLUSH can become just a bool that is
> returned by make_spte.
>
> - level and ad_disabled can be replaced by a single pointer to struct
> kvm_mmu_page (tdp_mmu does not set ad_disabled in page_role_for_level,
> but that's not an issue).
>
> - in mmu_set_spte, write_fault, speculative and host_writable are either
> false/true/true (prefetching) or fault->write, fault->prefault,
> fault->map_writable (pagefault).  So they can be replaced by a single
> struct kvm_page_fault pointer, where NULL means false/true/true.  Then
> if set_spte is inlined, the ugly bool arguments only remain in make_spte
> (minus ad_disabled).
>
> This does not remove the need for a separate slot parameter,
> but at least the balance is that there are no extra arguments to
> make_spte (two go, level and ad_disabled; two come, sp and slot).
>
> I've started hacking on the above, but didn't quite finish.  I'll
> keep patches 4-6 in my queue, but they'll have to wait for 5.15.

Ack. 5.15 sounds good. Let me know if you want any helping with testing.

>
> Paolo
>
> > ---
> >   arch/x86/kvm/mmu/mmu.c         | 29 ++++++++---------------------
> >   arch/x86/kvm/mmu/paging_tmpl.h | 12 +++++++++---
> >   2 files changed, 17 insertions(+), 24 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index c148d481e9b5..41e2ef8ad09b 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -1630,16 +1630,15 @@ static bool kvm_test_age_rmapp(struct kvm *kvm, struct kvm_rmap_head *rmap_head,
> >
> >   #define RMAP_RECYCLE_THRESHOLD 1000
> >
> > -static void rmap_add(struct kvm_vcpu *vcpu, u64 *spte, gfn_t gfn)
> > +static void rmap_add(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
> > +                  u64 *spte, gfn_t gfn)
> >   {
> > -     struct kvm_memory_slot *slot;
> >       struct kvm_mmu_page *sp;
> >       struct kvm_rmap_head *rmap_head;
> >       int rmap_count;
> >
> >       sp = sptep_to_sp(spte);
> >       kvm_mmu_page_set_gfn(sp, spte - sp->spt, gfn);
> > -     slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
> >       rmap_head = gfn_to_rmap(gfn, sp->role.level, slot);
> >       rmap_count = pte_list_add(vcpu, spte, rmap_head);
> >
> > @@ -2679,9 +2678,9 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
> >       return ret;
> >   }
> >
> > -static int mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
> > -                     unsigned int pte_access, bool write_fault, int level,
> > -                     gfn_t gfn, kvm_pfn_t pfn, bool speculative,
> > +static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
> > +                     u64 *sptep, unsigned int pte_access, bool write_fault,
> > +                     int level, gfn_t gfn, kvm_pfn_t pfn, bool speculative,
> >                       bool host_writable)
> >   {
> >       int was_rmapped = 0;
> > @@ -2744,24 +2743,12 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
> >
> >       if (!was_rmapped) {
> >               kvm_update_page_stats(vcpu->kvm, level, 1);
> > -             rmap_add(vcpu, sptep, gfn);
> > +             rmap_add(vcpu, slot, sptep, gfn);
> >       }
> >
> >       return ret;
> >   }
> >
> > -static kvm_pfn_t pte_prefetch_gfn_to_pfn(struct kvm_vcpu *vcpu, gfn_t gfn,
> > -                                  bool no_dirty_log)
> > -{
> > -     struct kvm_memory_slot *slot;
> > -
> > -     slot = gfn_to_memslot_dirty_bitmap(vcpu, gfn, no_dirty_log);
> > -     if (!slot)
> > -             return KVM_PFN_ERR_FAULT;
> > -
> > -     return gfn_to_pfn_memslot_atomic(slot, gfn);
> > -}
> > -
> >   static int direct_pte_prefetch_many(struct kvm_vcpu *vcpu,
> >                                   struct kvm_mmu_page *sp,
> >                                   u64 *start, u64 *end)
> > @@ -2782,7 +2769,7 @@ static int direct_pte_prefetch_many(struct kvm_vcpu *vcpu,
> >               return -1;
> >
> >       for (i = 0; i < ret; i++, gfn++, start++) {
> > -             mmu_set_spte(vcpu, start, access, false, sp->role.level, gfn,
> > +             mmu_set_spte(vcpu, slot, start, access, false, sp->role.level, gfn,
> >                            page_to_pfn(pages[i]), true, true);
> >               put_page(pages[i]);
> >       }
> > @@ -2979,7 +2966,7 @@ static int __direct_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> >                       account_huge_nx_page(vcpu->kvm, sp);
> >       }
> >
> > -     ret = mmu_set_spte(vcpu, it.sptep, ACC_ALL,
> > +     ret = mmu_set_spte(vcpu, fault->slot, it.sptep, ACC_ALL,
> >                          fault->write, fault->goal_level, base_gfn, fault->pfn,
> >                          fault->prefault, fault->map_writable);
> >       if (ret == RET_PF_SPURIOUS)
> > diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> > index 50ade6450ace..653ca44afa58 100644
> > --- a/arch/x86/kvm/mmu/paging_tmpl.h
> > +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> > @@ -561,6 +561,7 @@ static bool
> >   FNAME(prefetch_gpte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
> >                    u64 *spte, pt_element_t gpte, bool no_dirty_log)
> >   {
> > +     struct kvm_memory_slot *slot;
> >       unsigned pte_access;
> >       gfn_t gfn;
> >       kvm_pfn_t pfn;
> > @@ -573,8 +574,13 @@ FNAME(prefetch_gpte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
> >       gfn = gpte_to_gfn(gpte);
> >       pte_access = sp->role.access & FNAME(gpte_access)(gpte);
> >       FNAME(protect_clean_gpte)(vcpu->arch.mmu, &pte_access, gpte);
> > -     pfn = pte_prefetch_gfn_to_pfn(vcpu, gfn,
> > +
> > +     slot = gfn_to_memslot_dirty_bitmap(vcpu, gfn,
> >                       no_dirty_log && (pte_access & ACC_WRITE_MASK));
> > +     if (!slot)
> > +             return false;
> > +
> > +     pfn = gfn_to_pfn_memslot_atomic(slot, gfn);
> >       if (is_error_pfn(pfn))
> >               return false;
> >
> > @@ -582,7 +588,7 @@ FNAME(prefetch_gpte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
> >        * we call mmu_set_spte() with host_writable = true because
> >        * pte_prefetch_gfn_to_pfn always gets a writable pfn.
> >        */
> > -     mmu_set_spte(vcpu, spte, pte_access, false, PG_LEVEL_4K, gfn, pfn,
> > +     mmu_set_spte(vcpu, slot, spte, pte_access, false, PG_LEVEL_4K, gfn, pfn,
> >                    true, true);
> >
> >       kvm_release_pfn_clean(pfn);
> > @@ -749,7 +755,7 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
> >               }
> >       }
> >
> > -     ret = mmu_set_spte(vcpu, it.sptep, gw->pte_access, fault->write,
> > +     ret = mmu_set_spte(vcpu, fault->slot, it.sptep, gw->pte_access, fault->write,
> >                          it.level, base_gfn, fault->pfn, fault->prefault,
> >                          fault->map_writable);
> >       if (ret == RET_PF_SPURIOUS)
> >
>