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Subject[PATCH v2 2/4] KVM: x86/mmu: Expand quadrant comment for PG_LEVEL_4K shadow pages
From
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
Cc: David Matlack <dmatlack@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/mmu/mmu.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index b04e9ce2469a..83ca71361acd 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2166,7 +2166,8 @@ 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;
@@ -2193,9 +2194,15 @@ 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);
--
2.37.0.rc0.161.g10f37bed90-goog
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