Re: [PATCH 1/3] KVM: x86/mmu: Avoid subtle pointer arithmetic in kvm_mmu_child_role()

[David Matlack <dmatlack@google.com>]

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. 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;
        }