Re: [PATCH v2 03/26] KVM: x86/mmu: Derive shadow MMU page role from parent

Peter Xu
2022-03-15T08:15:34Z

On Fri, Mar 11, 2022 at 12:25:05AM +0000, David Matlack wrote:
> Instead of computing the shadow page role from scratch for every new
> page, we can derive most of the information from the parent shadow page.
> This avoids redundant calculations and reduces the number of parameters
> to kvm_mmu_get_page().
>
> Preemptively split out the role calculation to a separate function for
> use in a following commit.
>
> No functional change intended.
>
> Signed-off-by: David Matlack <dmatlack@google.com>

Looks right..

Reviewed-by: Peter Xu <peterx@redhat.com>

Two more comments/questions below.

> +static union kvm_mmu_page_role kvm_mmu_child_role(u64 *sptep, bool direct, u32 access)
> +{
> + struct kvm_mmu_page *parent_sp = sptep_to_sp(sptep);
> + union kvm_mmu_page_role role;
> +
> + role = parent_sp->role;
> + role.level--;
> + role.access = access;
> + role.direct = direct;
> +
> + /*
> + * If the guest has 4-byte PTEs then that means it's using 32-bit,
> + * 2-level, non-PAE paging. KVM shadows such guests using 4 PAE page
> + * directories, each mapping 1/4 of the guest's linear address space
> + * (1GiB). The shadow pages for those 4 page directories are
> + * pre-allocated and assigned a separate quadrant in their role.
> + *
> + * Since we are allocating a child shadow page and there are only 2
> + * levels, this must be a PG_LEVEL_4K shadow page. Here the quadrant
> + * will either be 0 or 1 because it maps 1/2 of the address space mapped
> + * by the guest's PG_LEVEL_4K page table (or 4MiB huge page) that it
> + * is shadowing. In this case, the quadrant can be derived by the index
> + * of the SPTE that points to the new child shadow page in the page
> + * directory (parent_sp). Specifically, every 2 SPTEs in parent_sp
> + * shadow one half of a guest's page table (or 4MiB huge page) so the
> + * quadrant is just the parity of the index of the SPTE.
> + */
> + if (role.has_4_byte_gpte) {
> + BUG_ON(role.level != PG_LEVEL_4K);
> + role.quadrant = (sptep - parent_sp->spt) % 2;
> + }

This made me wonder whether role.quadrant can be dropped, because it seems
it can be calculated out of the box with has_4_byte_gpte, level and spte
offset. I could have missed something, though..

> +
> + return role;
> +}
> +
> +static struct kvm_mmu_page *kvm_mmu_get_child_sp(struct kvm_vcpu *vcpu,
> + u64 *sptep, gfn_t gfn,
> + bool direct, u32 access)
> +{
> + union kvm_mmu_page_role role;
> +
> + role = kvm_mmu_child_role(sptep, direct, access);
> + return kvm_mmu_get_page(vcpu, gfn, role);

Nit: it looks nicer to just drop the temp var?

return kvm_mmu_get_page(vcpu, gfn,
kvm_mmu_child_role(sptep, direct, access));

Thanks,

--
Peter Xu