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Subject[PATCH v2] KVM: x86/MMU: Do not check unsync status for root SP.
Date
In shadow page table, only leaf SPs may be marked as unsync.
And for non-leaf SPs, we use unsync_children to keep the number
of the unsynced children. In kvm_mmu_sync_root(), sp->unsync
shall always be zero for the root SP, , hence no need to check
it. Instead, a warning inside mmu_sync_children() is added, in
case someone incorrectly used it.

Also, clarify the mmu_need_write_protect(), by moving the warning
into kvm_unsync_page().

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
Changes in V2:
- warnings added based on Sean's suggestion.

arch/x86/kvm/mmu/mmu.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 86af582..c4797a00cc 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -1995,6 +1995,12 @@ static void mmu_sync_children(struct kvm_vcpu *vcpu,
LIST_HEAD(invalid_list);
bool flush = false;

+ /*
+ * Only 4k SPTEs can directly be made unsync, the parent pages
+ * should never be unsyc'd.
+ */
+ WARN_ON_ONCE(sp->unsync);
+
while (mmu_unsync_walk(parent, &pages)) {
bool protected = false;

@@ -2502,6 +2508,8 @@ int kvm_mmu_unprotect_page(struct kvm *kvm, gfn_t gfn)

static void kvm_unsync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
{
+ WARN_ON(sp->role.level != PG_LEVEL_4K);
+
trace_kvm_mmu_unsync_page(sp);
++vcpu->kvm->stat.mmu_unsync;
sp->unsync = 1;
@@ -2524,7 +2532,6 @@ bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
if (sp->unsync)
continue;

- WARN_ON(sp->role.level != PG_LEVEL_4K);
kvm_unsync_page(vcpu, sp);
}

@@ -3406,8 +3413,7 @@ void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
* mmu_need_write_protect() describe what could go wrong if this
* requirement isn't satisfied.
*/
- if (!smp_load_acquire(&sp->unsync) &&
- !smp_load_acquire(&sp->unsync_children))
+ if (!smp_load_acquire(&sp->unsync_children))
return;

write_lock(&vcpu->kvm->mmu_lock);
--
1.9.1
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