hw/virtio/vhost: re-factor vhost-section and allow DIRTY_MEMORY_CODE

The purpose of vhost_section is to identify RAM regions that need to
be made available to a vhost client. However when running under TCG
all RAM sections have DIRTY_MEMORY_CODE set which leads to problems
down the line.

Re-factor the code so:

  - steps are clearer to follow
  - reason for rejection is recorded in the trace point
  - we allow DIRTY_MEMORY_CODE

We expand the comment to explain that kernel based vhost has specific
support for migration tracking.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Fabiano Rosas <farosas@linux.ibm.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20200605154929.26910-11-alex.bennee@linaro.org>

1 files changed, 41 insertions, 16 deletions

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index aff98a0ede..e3e2181290 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -27,6 +27,7 @@
 #include "migration/blocker.h"
 #include "migration/qemu-file-types.h"
 #include "sysemu/dma.h"
+#include "sysemu/tcg.h"
 #include "trace.h"
 
 /* enabled until disconnected backend stabilizes */
@@ -403,26 +404,50 @@ static int vhost_verify_ring_mappings(struct vhost_dev *dev,
     return r;
 }
 
+/*
+ * vhost_section: identify sections needed for vhost access
+ *
+ * We only care about RAM sections here (where virtqueue and guest
+ * internals accessed by virtio might live). If we find one we still
+ * allow the backend to potentially filter it out of our list.
+ */
 static bool vhost_section(struct vhost_dev *dev, MemoryRegionSection *section)
 {
-    bool result;
-    bool log_dirty = memory_region_get_dirty_log_mask(section->mr) &
-                     ~(1 << DIRTY_MEMORY_MIGRATION);
-    result = memory_region_is_ram(section->mr) &&
-        !memory_region_is_rom(section->mr);
-
-    /* Vhost doesn't handle any block which is doing dirty-tracking other
-     * than migration; this typically fires on VGA areas.
-     */
-    result &= !log_dirty;
+    MemoryRegion *mr = section->mr;
+
+    if (memory_region_is_ram(mr) && !memory_region_is_rom(mr)) {
+        uint8_t dirty_mask = memory_region_get_dirty_log_mask(mr);
+        uint8_t handled_dirty;
+
+        /*
+         * Kernel based vhost doesn't handle any block which is doing
+         * dirty-tracking other than migration for which it has
+         * specific logging support. However for TCG the kernel never
+         * gets involved anyway so we can also ignore it's
+         * self-modiying code detection flags. However a vhost-user
+         * client could still confuse a TCG guest if it re-writes
+         * executable memory that has already been translated.
+         */
+        handled_dirty = (1 << DIRTY_MEMORY_MIGRATION) |
+            (1 << DIRTY_MEMORY_CODE);
 
-    if (result && dev->vhost_ops->vhost_backend_mem_section_filter) {
-        result &=
-            dev->vhost_ops->vhost_backend_mem_section_filter(dev, section);
-    }
+        if (dirty_mask & ~handled_dirty) {
+            trace_vhost_reject_section(mr->name, 1);
+            return false;
+        }
+
+        if (dev->vhost_ops->vhost_backend_mem_section_filter &&
+            !dev->vhost_ops->vhost_backend_mem_section_filter(dev, section)) {
+            trace_vhost_reject_section(mr->name, 2);
+            return false;
+        }
 
-    trace_vhost_section(section->mr->name, result);
-    return result;
+        trace_vhost_section(mr->name);
+        return true;
+    } else {
+        trace_vhost_reject_section(mr->name, 3);
+        return false;
+    }
 }
 
 static void vhost_begin(MemoryListener *listener)