1 files changed, 33 insertions, 0 deletions
diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 3842e70bdb7c..b982112d2ca5 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -104,6 +104,39 @@ static inline int is_present_gpte(unsigned long pte)
 	return pte & PT_PRESENT_MASK;
 }
 
+/*
+ * Currently, we have two sorts of write-protection, a) the first one
+ * write-protects guest page to sync the guest modification, b) another one is
+ * used to sync dirty bitmap when we do KVM_GET_DIRTY_LOG. The differences
+ * between these two sorts are:
+ * 1) the first case clears SPTE_MMU_WRITEABLE bit.
+ * 2) the first case requires flushing tlb immediately avoiding corrupting
+ *    shadow page table between all vcpus so it should be in the protection of
+ *    mmu-lock. And the another case does not need to flush tlb until returning
+ *    the dirty bitmap to userspace since it only write-protects the page
+ *    logged in the bitmap, that means the page in the dirty bitmap is not
+ *    missed, so it can flush tlb out of mmu-lock.
+ *
+ * So, there is the problem: the first case can meet the corrupted tlb caused
+ * by another case which write-protects pages but without flush tlb
+ * immediately. In order to making the first case be aware this problem we let
+ * it flush tlb if we try to write-protect a spte whose SPTE_MMU_WRITEABLE bit
+ * is set, it works since another case never touches SPTE_MMU_WRITEABLE bit.
+ *
+ * Anyway, whenever a spte is updated (only permission and status bits are
+ * changed) we need to check whether the spte with SPTE_MMU_WRITEABLE becomes
+ * readonly, if that happens, we need to flush tlb. Fortunately,
+ * mmu_spte_update() has already handled it perfectly.
+ *
+ * The rules to use SPTE_MMU_WRITEABLE and PT_WRITABLE_MASK:
+ * - if we want to see if it has writable tlb entry or if the spte can be
+ *   writable on the mmu mapping, check SPTE_MMU_WRITEABLE, this is the most
+ *   case, otherwise
+ * - if we fix page fault on the spte or do write-protection by dirty logging,
+ *   check PT_WRITABLE_MASK.
+ *
+ * TODO: introduce APIs to split these two cases.
+ */
 static inline int is_writable_pte(unsigned long pte)
 {
 	return pte & PT_WRITABLE_MASK;