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@xxxxxxxxxx Cc: David Matlack <dmatlack@xxxxxxxxxx> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> --- 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
