[PATCH 6/7] KVM: MMU: clarify fast_pf_fix_direct_spte

[guangrong.xiao@gmail.com]

From: Xiao Guangrong <xiaoguangrong@tencent.com>

The writable spte can not be locklessly fixed and add a WARN_ON()
to trigger the warning if something out of our mind happens, that
is good for us to track if the log for writable spte is missed
on the fast path

Signed-off-by: Xiao Guangrong <xiaoguangrong@tencent.com>
---
 arch/x86/kvm/mmu.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index ad6ee46..f6a74e7 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -3225,6 +3225,15 @@ fast_pf_fix_direct_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 	WARN_ON(!sp->role.direct);
 
 	/*
+	 * the original spte can not be writable as only the spte which
+	 * fulfills is_access_track_spte() or
+	 * spte_can_locklessly_be_made_writable() can be locklessly fixed,
+	 * for the former, the W bit is always cleared, for the latter,
+	 * there is nothing to do if it is already writable.
+	 */
+	WARN_ON(is_writable_pte(old_spte));
+
+	/*
 	 * Theoretically we could also set dirty bit (and flush TLB) here in
 	 * order to eliminate unnecessary PML logging. See comments in
 	 * set_spte. But fast_page_fault is very unlikely to happen with PML
@@ -3239,7 +3248,7 @@ fast_pf_fix_direct_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 	if (cmpxchg64(sptep, old_spte, new_spte) != old_spte)
 		return false;
 
-	if (is_writable_pte(new_spte) && !is_writable_pte(old_spte)) {
+	if (is_writable_pte(new_spte)) {
 		/*
 		 * The gfn of direct spte is stable since it is
 		 * calculated by sp->gfn.
-- 
2.9.3