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-rw-r--r--Documentation/kvm/mmu.txt19
1 files changed, 19 insertions, 0 deletions
diff --git a/Documentation/kvm/mmu.txt b/Documentation/kvm/mmu.txt
index 2201dcba92a1..1e7ecdd15bba 100644
--- a/Documentation/kvm/mmu.txt
+++ b/Documentation/kvm/mmu.txt
@@ -298,6 +298,25 @@ Host translation updates:
- look up affected sptes through reverse map
- drop (or update) translations
+Emulating cr0.wp
+================
+
+If tdp is not enabled, the host must keep cr0.wp=1 so page write protection
+works for the guest kernel, not guest guest userspace. When the guest
+cr0.wp=1, this does not present a problem. However when the guest cr0.wp=0,
+we cannot map the permissions for gpte.u=1, gpte.w=0 to any spte (the
+semantics require allowing any guest kernel access plus user read access).
+
+We handle this by mapping the permissions to two possible sptes, depending
+on fault type:
+
+- kernel write fault: spte.u=0, spte.w=1 (allows full kernel access,
+ disallows user access)
+- read fault: spte.u=1, spte.w=0 (allows full read access, disallows kernel
+ write access)
+
+(user write faults generate a #PF)
+
Further reading
===============