KVM: PPC: Disable NX for old magic page using guests

Old guests try to use the magic page, but map their trampoline code inside
of an NX region.

Since we can't fix those old kernels, try to detect whether the guest is sane
or not. If not, just disable NX functionality in KVM so that old guests at
least work at all. For newer guests, add a bit that we can set to keep NX
functionality available.

Signed-off-by: Alexander Graf <agraf@suse.de>

1 files changed, 14 insertions, 0 deletions

diff --git a/Documentation/virtual/kvm/ppc-pv.txt b/Documentation/virtual/kvm/ppc-pv.txt
index 4643cde517c4..319560646f32 100644
--- a/Documentation/virtual/kvm/ppc-pv.txt
+++ b/Documentation/virtual/kvm/ppc-pv.txt
@@ -94,10 +94,24 @@ a bitmap of available features inside the magic page.
 The following enhancements to the magic page are currently available:
 
   KVM_MAGIC_FEAT_SR             Maps SR registers r/w in the magic page
+  KVM_MAGIC_FEAT_MAS0_TO_SPRG7  Maps MASn, ESR, PIR and high SPRGs
 
 For enhanced features in the magic page, please check for the existence of the
 feature before using them!
 
+Magic page flags
+================
+
+In addition to features that indicate whether a host is capable of a particular
+feature we also have a channel for a guest to tell the guest whether it's capable
+of something. This is what we call "flags".
+
+Flags are passed to the host in the low 12 bits of the Effective Address.
+
+The following flags are currently available for a guest to expose:
+
+  MAGIC_PAGE_FLAG_NOT_MAPPED_NX Guest handles NX bits correclty wrt magic page
+
 MSR bits
 ========