libxl: keep PoD target adjustment by memory fudge after reload_domain_config()

Commit 56fb5fd623 ("libxl: adjust PoD target by memory fudge, too")
introduced target_memkb adjustment for HVM PoD domains on create,
wherein the value it wrote to target is always 1MiB lower than the
actual target_memkb.  Unfortunately, on reboot, it is this value which
is read *unmodified* to feed into the next domain creation; from which
1MiB is subtracted *again*.  This means that any guest which reboots
with memory < maxmem will have its memory target decreased by 1MiB on
every boot.

This patch makes it so that when reading target on reboot, we adjust the
value we read *up* by 1MiB, so that the domain will be build with the
appropriate amount of memory and the target will remain the same after
reboot.

This is still not quite a complete fix, as the 1MiB offset is only
subtracted when creating or rebooting; it is not subtracted when 'xl
set-memory' is called.  But it will prevent any situations where memory
is continually increased or decreased.  A better fix will have to wait
until after the release.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>

tools/libxl/libxl.c

@@ -7229,12 +7229,12 @@ int libxl_retrieve_domain_configuration(libxl_ctx *ctx, uint32_t domid,
             LOG(ERROR, "fail to get memory target for domain %d", domid);
             goto out;
         }
-        /* Target memory in xenstore is different from what user has
-         * asked for. The difference is video_memkb. See
-         * libxl_set_memory_target.
+
+        /* libxl__get_targetmem_fudge() calculates the difference from
+         * what is in xenstore to what we have in the domain build info.
          */
         d_config->b_info.target_memkb = target_memkb +
-            d_config->b_info.video_memkb;
+            libxl__get_targetmem_fudge(gc, &d_config->b_info);
 
         d_config->b_info.max_memkb = max_memkb;
     }

tools/libxl/libxl_dom.c

@@ -490,7 +490,6 @@ int libxl__build_post(libxl__gc *gc, uint32_t domid,
     xs_transaction_t t;
     char **ents;
     int i, rc;
-    int64_t mem_target_fudge;
 
     if (info->num_vnuma_nodes && !info->num_vcpu_soft_affinity) {
         rc = set_vnuma_affinity(gc, domid, info);
@@ -523,17 +522,12 @@ int libxl__build_post(libxl__gc *gc, uint32_t domid,
         }
     }
 
-    mem_target_fudge =
-        (info->type == LIBXL_DOMAIN_TYPE_HVM &&
-         info->max_memkb > info->target_memkb)
-        ? LIBXL_MAXMEM_CONSTANT : 0;
-
     ents = libxl__calloc(gc, 12 + (info->max_vcpus * 2) + 2, sizeof(char *));
     ents[0] = "memory/static-max";
     ents[1] = GCSPRINTF("%"PRId64, info->max_memkb);
     ents[2] = "memory/target";
-    ents[3] = GCSPRINTF("%"PRId64, info->target_memkb - info->video_memkb
-                        - mem_target_fudge);
+    ents[3] = GCSPRINTF("%"PRId64, info->target_memkb -
+                        libxl__get_targetmem_fudge(gc, info));
     ents[4] = "memory/videoram";
     ents[5] = GCSPRINTF("%"PRId64, info->video_memkb);
     ents[6] = "domid";

tools/libxl/libxl_internal.h

@@ -4107,6 +4107,21 @@ static inline void libxl__update_config_vtpm(libxl__gc *gc,
     libxl_uuid_copy(CTX, &dst->uuid, &src->uuid);
 }
 
+/* Target memory in xenstore is different from what user has
+ * asked for. The difference is video_memkb + (possible) fudge.
+ * See libxl_set_memory_target.
+ */
+static inline
+uint64_t libxl__get_targetmem_fudge(libxl__gc *gc,
+                                    const libxl_domain_build_info *info)
+{
+    int64_t mem_target_fudge = (info->type == LIBXL_DOMAIN_TYPE_HVM &&
+                                info->max_memkb > info->target_memkb)
+                                ? LIBXL_MAXMEM_CONSTANT : 0;
+
+    return info->video_memkb + mem_target_fudge;
+}
+
 /* Macros used to compare device identifier. Returns true if the two
  * devices have same identifier. */
 #define COMPARE_DEVID(a, b) ((a)->devid == (b)->devid)