[PATCH v4 05/20] KVM: x86/mmu: Consolidate shadow page allocation and initialization

[David Matlack]

On Thu, May 5, 2022 at 3:10 PM Sean Christopherson <seanjc at google.com> wrote:
>
> On Fri, Apr 22, 2022, David Matlack wrote:
> > Consolidate kvm_mmu_alloc_page() and kvm_mmu_alloc_shadow_page() under
> > the latter so that all shadow page allocation and initialization happens
> > in one place.
> >
> > No functional change intended.
> >
> > Signed-off-by: David Matlack <dmatlack at google.com>
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 39 +++++++++++++++++----------------------
> >  1 file changed, 17 insertions(+), 22 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 5582badf4947..7d03320f6e08 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -1703,27 +1703,6 @@ static void drop_parent_pte(struct kvm_mmu_page *sp,
> >       mmu_spte_clear_no_track(parent_pte);
> >  }
> >
> > -static struct kvm_mmu_page *kvm_mmu_alloc_page(struct kvm_vcpu *vcpu, bool direct)
> > -{
> > -     struct kvm_mmu_page *sp;
> > -
> > -     sp = kvm_mmu_memory_cache_alloc(&vcpu->arch.mmu_page_header_cache);
> > -     sp->spt = kvm_mmu_memory_cache_alloc(&vcpu->arch.mmu_shadow_page_cache);
> > -     if (!direct)
> > -             sp->gfns = kvm_mmu_memory_cache_alloc(&vcpu->arch.mmu_gfn_array_cache);
> > -     set_page_private(virt_to_page(sp->spt), (unsigned long)sp);
> > -
> > -     /*
> > -      * active_mmu_pages must be a FIFO list, as kvm_zap_obsolete_pages()
> > -      * depends on valid pages being added to the head of the list.  See
> > -      * comments in kvm_zap_obsolete_pages().
> > -      */
> > -     sp->mmu_valid_gen = vcpu->kvm->arch.mmu_valid_gen;
> > -     list_add(&sp->link, &vcpu->kvm->arch.active_mmu_pages);
> > -     kvm_mod_used_mmu_pages(vcpu->kvm, +1);
> > -     return sp;
> > -}
> > -
> >  static void mark_unsync(u64 *spte);
> >  static void kvm_mmu_mark_parents_unsync(struct kvm_mmu_page *sp)
> >  {
> > @@ -2100,7 +2079,23 @@ static struct kvm_mmu_page *kvm_mmu_alloc_shadow_page(struct kvm_vcpu *vcpu,
> >                                                     struct hlist_head *sp_list,
> >                                                     union kvm_mmu_page_role role)
> >  {
> > -     struct kvm_mmu_page *sp = kvm_mmu_alloc_page(vcpu, role.direct);
> > +     struct kvm_mmu_page *sp;
> > +
> > +     sp = kvm_mmu_memory_cache_alloc(&vcpu->arch.mmu_page_header_cache);
> > +     sp->spt = kvm_mmu_memory_cache_alloc(&vcpu->arch.mmu_shadow_page_cache);
> > +     if (!role.direct)
> > +             sp->gfns = kvm_mmu_memory_cache_alloc(&vcpu->arch.mmu_gfn_array_cache);
> > +
> > +     set_page_private(virt_to_page(sp->spt), (unsigned long)sp);
> > +
> > +     /*
> > +      * active_mmu_pages must be a FIFO list, as kvm_zap_obsolete_pages()
> > +      * depends on valid pages being added to the head of the list.  See
> > +      * comments in kvm_zap_obsolete_pages().
> > +      */
> > +     sp->mmu_valid_gen = vcpu->kvm->arch.mmu_valid_gen;
> > +     list_add(&sp->link, &vcpu->kvm->arch.active_mmu_pages);
> > +     kvm_mod_used_mmu_pages(vcpu->kvm, +1);
>
> To reduce churn later on, what about opportunistically grabbing vcpu->kvm in a
> local variable in this patch?
>
> Actually, at that point, it's a super trivial change, so you can probably just drop
>
>         KVM: x86/mmu: Replace vcpu with kvm in kvm_mmu_alloc_shadow_page()
>
> and do the vcpu/kvm swap as part of
>
>         KVM: x86/mmu: Pass memory caches to allocate SPs separately

I'm not sure it's any less churn, it's just doing the same amount of
changes in fewer commits. Is there a benefit of using less commits? I
can only think of downsides (harder to review, harder to bisect).

>
> >       sp->gfn = gfn;
> >       sp->role = role;
> > --
> > 2.36.0.rc2.479.g8af0fa9b8e-goog
> >