Commit 39944ab9 authored by Sean Christopherson's avatar Sean Christopherson Committed by Paolo Bonzini
Browse files

KVM: x86/mmu: Expand quadrant comment for PG_LEVEL_4K shadow pages 

Tweak the comment above the computation of the quadrant for PG_LEVEL_4K
shadow pages to explicitly call out how and why KVM uses role.quadrant to
consume gPTE bits.

Opportunistically wrap an unnecessarily long line.

No functional change intended.

Link: https://lore.kernel.org/all/YqvWvBv27fYzOFdE@google.com


Reviewed-by: default avatarDavid Matlack <dmatlack@google.com>
Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
Message-Id: <20220712020724.1262121-3-seanjc@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 79e48cec

arch/x86/kvm/mmu/mmu.c

+11 −4
Original line number Diff line number Diff line
@@ -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;
@@ -2195,9 +2196,15 @@ 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);