Re: [PATCH V2 3/7] KVM: X86/MMU: Link PAE root pagetable with its children

[David Matlack <dmatlack@google.com>]

On Tue, May 17, 2022 at 09:13:54AM +0800, Lai Jiangshan wrote:
> On Tue, May 17, 2022 at 8:01 AM David Matlack <dmatlack@google.com> wrote:
> >
> > On Tue, May 03, 2022 at 11:07:31PM +0800, Lai Jiangshan wrote:
> > > From: Lai Jiangshan <jiangshan.ljs@antgroup.com>
> > >
> > > When special shadow pages are activated, link_shadow_page() might link
> > > a special shadow pages which is the PAE root for PAE paging with its
> > > children.
> > >
> > > Add make_pae_pdpte() to handle it.
> > >
> > > The code is not activated since special shadow pages are not activated
> > > yet.
> > >
> > > Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
> > > ---
> > >  arch/x86/kvm/mmu/mmu.c  | 6 +++++-
> > >  arch/x86/kvm/mmu/spte.c | 7 +++++++
> > >  arch/x86/kvm/mmu/spte.h | 1 +
> > >  3 files changed, 13 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index 126f0cd07f98..3fe70ad3bda2 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -2277,7 +2277,11 @@ static void link_shadow_page(struct kvm_vcpu *vcpu, u64 *sptep,
> > >
> > >       BUILD_BUG_ON(VMX_EPT_WRITABLE_MASK != PT_WRITABLE_MASK);
> > >
> > > -     spte = make_nonleaf_spte(sp->spt, sp_ad_disabled(sp));
> > > +     if (unlikely(sp->role.level == PT32_ROOT_LEVEL &&
> > > +                  vcpu->arch.mmu->root_role.level == PT32E_ROOT_LEVEL))
> > > +             spte = make_pae_pdpte(sp->spt);
> > > +     else
> > > +             spte = make_nonleaf_spte(sp->spt, sp_ad_disabled(sp));
> > >
> > >       mmu_spte_set(sptep, spte);
> > >
> > > diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
> > > index 75c9e87d446a..ccd9267a58ca 100644
> > > --- a/arch/x86/kvm/mmu/spte.c
> > > +++ b/arch/x86/kvm/mmu/spte.c
> > > @@ -251,6 +251,13 @@ u64 make_huge_page_split_spte(u64 huge_spte, int huge_level, int index)
> > >       return child_spte;
> > >  }
> > >
> > > +u64 make_pae_pdpte(u64 *child_pt)
> > > +{
> > > +     /* The only ignore bits in PDPTE are 11:9. */
> > > +     BUILD_BUG_ON(!(GENMASK(11,9) & SPTE_MMU_PRESENT_MASK));
> > > +     return __pa(child_pt) | PT_PRESENT_MASK | SPTE_MMU_PRESENT_MASK |
> > > +             shadow_me_value;
> >
> > If I'm reading mmu_alloc_{direct,shadow}_roots() correctly, PAE page
> > directories just get: root | PT_PRESENT_MASK | shadow_me_value. Is there
> > a reason to add SPTE_MMU_PRESENT_MASK or am I misreading the code?
> 
> Because it has a struct kvm_mmu_page associated with it now.
> 
> sp->spt[i] requires SPTE_MMU_PRESENT_MASK if it is present.

Ah of course. e.g. FNAME(fetch) will call is_shadow_present_pte() on PAE
PDPTEs.

Could you also update the comment above SPTE_MMU_PRESENT_MASK? Right now it
says: "Use bit 11, as it is ignored by all flavors of SPTEs and checking a low
bit often generates better code than for a high bit, e.g. 56+." I think it
would be helpful to also meniton that SPTE_MMU_PRESENT_MASK is also used in
PDPTEs which only ignore bits 11:9.


> 
> >
> > > +}
> > >
> > >  u64 make_nonleaf_spte(u64 *child_pt, bool ad_disabled)
> > >  {
> > > diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> > > index fbbab180395e..09a7e4ba017a 100644
> > > --- a/arch/x86/kvm/mmu/spte.h
> > > +++ b/arch/x86/kvm/mmu/spte.h
> > > @@ -413,6 +413,7 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
> > >              u64 old_spte, bool prefetch, bool can_unsync,
> > >              bool host_writable, u64 *new_spte);
> > >  u64 make_huge_page_split_spte(u64 huge_spte, int huge_level, int index);
> > > +u64 make_pae_pdpte(u64 *child_pt);
> > >  u64 make_nonleaf_spte(u64 *child_pt, bool ad_disabled);
> > >  u64 make_mmio_spte(struct kvm_vcpu *vcpu, u64 gfn, unsigned int access);
> > >  u64 mark_spte_for_access_track(u64 spte);
> > > --
> > > 2.19.1.6.gb485710b
> > >