[PATCH 3/3] KVM: MMU: Document cr0.wp emulation

[Avi Kivity <avi@redhat.com>]

Signed-off-by: Avi Kivity <avi@redhat.com>
---
 Documentation/kvm/mmu.txt |   18 ++++++++++++++++++
 1 files changed, 18 insertions(+), 0 deletions(-)

diff --git a/Documentation/kvm/mmu.txt b/Documentation/kvm/mmu.txt
index 2201dcb..6528ad9 100644
--- a/Documentation/kvm/mmu.txt
+++ b/Documentation/kvm/mmu.txt
@@ -298,6 +298,24 @@ 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.
+
+We handle this my mapping the permissions to two possible gptes, 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
 ===============
 
-- 
1.7.1