[Qemu-devel] [PATCH 13/20] qom-fuse: workaround for truncated properties > 4096

[Michael Roth <mdroth@linux.vnet.ibm.com>]

We currently hard-code property size at 4096 for the purposes of
getattr()/stat()/etc. For 'state' properties we can exceed this easily,
leading to truncated responses.

Instead, for a particular property, make it
max(4096, most_recent_property_size * 2). This allows some
head-room for properties that change size periodically (numbers,
strings, state properties containing arrays, etc)

Also, implement a simple property cache to avoid spinning on qom-get
if an application reads beyond the actual size. This also allows us
to use a snapshot of a single qom-get that persists across read()'s.
Old Cache entries are evicted as soon as we attempt to read() from
offset 0 again.

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
---
 QMP/qom-fuse |   24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/QMP/qom-fuse b/QMP/qom-fuse
index 5c6754a..bd43f29 100755
--- a/QMP/qom-fuse
+++ b/QMP/qom-fuse
@@ -26,6 +26,7 @@ class QOMFS(Fuse):
         self.qmp.connect()
         self.ino_map = {}
         self.ino_count = 1
+        self.prop_cache = {}
 
     def get_ino(self, path):
         if self.ino_map.has_key(path):
@@ -67,12 +68,16 @@ class QOMFS(Fuse):
         if not self.is_property(path):
             return -ENOENT
 
-        path, prop = path.rsplit('/', 1)
-        try:
-            data = str(self.qmp.command('qom-get', path=path, property=prop))
-            data += '\n' # make values shell friendly
-        except:
-            return -EPERM
+        # avoid extra calls to qom-get by using cached value when offset > 0
+        if offset == 0 or not self.prop_cache.has_key(path):
+            directory, prop = path.rsplit('/', 1)
+            try:
+                resp = str(self.qmp.command('qom-get', path=directory, property=prop))
+                self.prop_cache[path] = resp + '\n' # make values shell friendly
+            except:
+                return -EPERM
+
+        data = self.prop_cache[path]
 
         if offset > len(data):
             return ''
@@ -111,13 +116,18 @@ class QOMFS(Fuse):
                                        0,
                                        0))
         elif self.is_property(path):
+            directory, prop = path.rsplit('/', 1)
+            try:
+                resp = str(self.qmp.command('qom-get', path=directory, property=prop))
+            except:
+                return -ENOENT
             value = posix.stat_result((0644 | stat.S_IFREG,
                                        self.get_ino(path),
                                        0,
                                        1,
                                        1000,
                                        1000,
-                                       4096,
+                                       max(len(resp) * 2, 4096),
                                        0,
                                        0,
                                        0))
-- 
1.7.9.5