[PATCH v2 7/8] KVM: s390: vsie: Fix guest page tables protection

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

When shadowing, the guest page tables are write-protected, in order to
trap changes and properly unshadow the shadow mapping for the nested
guest. Already shadowed levels are skipped, so that only the needed
levels are write protected.

Currently the levels that get write protected are exactly one level too
deep: the last level (nested guest memory) gets protected in the wrong
way, and will be protected again correctly a few lines afterwards; most
importantly, the highest non-shadowed level does *not* get write
protected.

Moreover, if the nested guest is running in a real address space, there
are no DAT tables to shadow.

Write protect the correct levels, so that all the levels that need to
be protected are protected, and avoid double protecting the last level;
skip attempting to shadow the DAT tables when the nested guest is
running in a real address space.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Fixes: e38c884df921 ("KVM: s390: Switch to new gmap")
---
 arch/s390/kvm/gaccess.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/arch/s390/kvm/gaccess.c b/arch/s390/kvm/gaccess.c
index 1054f9bd107f..17563f889c6b 100644
--- a/arch/s390/kvm/gaccess.c
+++ b/arch/s390/kvm/gaccess.c
@@ -1517,13 +1517,15 @@ static int _gaccess_do_shadow(struct kvm_s390_mmu_cache *mc, struct gmap *sg,
 	 * Skip levels that are already protected. For each level, protect
 	 * only the page containing the entry, not the whole table.
 	 */
-	for (i = gl ; i >= w->level; i--) {
-		rc = gmap_protect_rmap(mc, sg, entries[i - 1].gfn, gpa_to_gfn(saddr),
-				       entries[i - 1].pfn, i, entries[i - 1].writable);
-		if (rc)
-			return rc;
-		if (!sg->parent)
-			return -EAGAIN;
+	if (w->level > LEVEL_MEM) {
+		for (i = gl ; i >= w->level; i--) {
+			rc = gmap_protect_rmap(mc, sg, entries[i].gfn, gpa_to_gfn(saddr),
+					       entries[i].pfn, i + 1, entries[i].writable);
+			if (rc)
+				return rc;
+			if (!sg->parent)
+				return -EAGAIN;
+		}
 	}
 
 	rc = dat_entry_walk(NULL, entries[LEVEL_MEM].gfn, sg->parent->asce, DAT_WALK_LEAF,
-- 
2.53.0