From: Avi Kivity <avi@redhat.com>
Date: Thu, 27 May 2010 11:46:04 +0000 (+0300)
Subject: KVM: MMU: Document cr0.wp emulation
X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=ec87fe2afcbcc4f430554980ec3e408bae34229d;p=linux-beck.git

KVM: MMU: Document cr0.wp emulation

Signed-off-by: Avi Kivity <avi@redhat.com>
---

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
 ===============