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SubjectRe: [PATCH 1/3] KVM: x86/mmu: Avoid subtle pointer arithmetic in kvm_mmu_child_role()
On Fri, Jun 24, 2022, David Matlack wrote:
> On Fri, Jun 24, 2022 at 05:18:06PM +0000, Sean Christopherson wrote:
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -2168,7 +2168,8 @@ static struct kvm_mmu_page *kvm_mmu_get_shadow_page(struct kvm_vcpu *vcpu,
> > return __kvm_mmu_get_shadow_page(vcpu->kvm, vcpu, &caches, gfn, role);
> > }
> >
> > -static union kvm_mmu_page_role kvm_mmu_child_role(u64 *sptep, bool direct, unsigned int access)
> > +static union kvm_mmu_page_role kvm_mmu_child_role(u64 *sptep, bool direct,
> > + unsigned int access)
> > {
> > struct kvm_mmu_page *parent_sp = sptep_to_sp(sptep);
> > union kvm_mmu_page_role role;
> > @@ -2195,13 +2196,19 @@ static union kvm_mmu_page_role kvm_mmu_child_role(u64 *sptep, bool direct, unsig
> > * uses 2 PAE page tables, each mapping a 2MiB region. For these,
> > * @role.quadrant encodes which half of the region they map.
> > *
> > - * Note, the 4 PAE page directories are pre-allocated and the quadrant
> > - * assigned in mmu_alloc_root(). So only page tables need to be handled
> > - * here.
> > + * Concretely, a 4-byte PDE consumes bits 31:22, while an 8-byte PDE
> > + * consumes bits 29:21. To consume bits 31:30, KVM's uses 4 shadow
> > + * PDPTEs; those 4 PAE page directories are pre-allocated and their
> > + * quadrant is assigned in mmu_alloc_root(). A 4-byte PTE consumes
> > + * bits 21:12, while an 8-byte PTE consumes bits 20:12. To consume
> > + * bit 21 in the PTE (the child here), KVM propagates that bit to the
> > + * quadrant, i.e. sets quadrant to '0' or '1'. The parent 8-byte PDE
> > + * covers bit 21 (see above), thus the quadrant is calculated from the
> > + * _least_ significant bit of the PDE index.
> > */
> > if (role.has_4_byte_gpte) {
> > WARN_ON_ONCE(role.level != PG_LEVEL_4K);
> > - role.quadrant = (sptep - parent_sp->spt) % 2;
> > + role.quadrant = ((unsigned long)sptep / sizeof(*sptep)) & 1;
> > }
>
> I find both difficult to read TBH.

No argument there. My objection to the pointer arithmetic is that it's easy to
misread.

> And "sptep -> sp->spt" is repeated in other places.

>
> How about using this oppotunity to introduce a helper that turns an
> sptep into an index to use here and clean up the other users?
>
> e.g.
>
> static inline int spte_index(u64 *sptep)
> {
> return ((unsigned long)sptep / sizeof(*sptep)) & (SPTE_ENT_PER_PAGE - 1);
> }
>
> Then kvm_mmu_child_role() becomes:
>
> if (role.has_4_byte_gpte) {
> WARN_ON_ONCE(role.level != PG_LEVEL_4K);
> role.quadrant = spte_index(sptep) & 1;
> }

Nice! I like this a lot. Will do in v2.

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