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

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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: David Matlack <dmatlack@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220712020724.1262121-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2022-07-12 02:07:23 +00:00 committed by Paolo Bonzini
parent 79e48cec6c
commit 39944ab99c
1 changed files with 11 additions and 4 deletions
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15
arch/x86/kvm/mmu/mmu.c
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@ -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);