[PATCH v3 2/2] tools: allow to limit xenstore features via guest config

[Juergen Gross <jgross@xxxxxxxx>]

Add a guest config parameter "xenstore_feature_mask" allowing to limit
the Xenstore features the guest can see and use. This can be needed in
order to allow migrating a guest to a host running a Xenstore version
providing less features than the source host.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Jason Andryuk <jason.andryuk@xxxxxxx>
Acked-by: Nick Rosbrook <enr0n@xxxxxxxxxx> # tools/golang
---
V2:
- fix typo (Jason Andryuk)
V3:
- use LOGED for error logging (Anthony Perard)
- don't leak vm_path in case of error (Anthony Perard)
---
 docs/man/xl.cfg.5.pod.in             | 36 ++++++++++++++++++++++++++++
 tools/golang/xenlight/helpers.gen.go |  2 ++
 tools/golang/xenlight/types.gen.go   |  1 +
 tools/include/libxl.h                |  6 +++++
 tools/libs/light/libxl_dom.c         | 17 ++++++++++++-
 tools/libs/light/libxl_types.idl     |  1 +
 tools/xl/xl_parse.c                  |  3 +++
 7 files changed, 65 insertions(+), 1 deletion(-)

diff --git a/docs/man/xl.cfg.5.pod.in b/docs/man/xl.cfg.5.pod.in
index 5362fb0e9a..f0c920b39d 100644
--- a/docs/man/xl.cfg.5.pod.in
+++ b/docs/man/xl.cfg.5.pod.in
@@ -714,6 +714,42 @@ If this option is not specified then it will default to 
B<false>.
 
 =back
 
+=head3 Xenstore related settings
+
+=over 4
+
+=item B<xenstore_feature_mask=NUMBER>
+
+Specify which Xenstore features are visible for the guest.
+
+This might be needed when a guest should be able to be migrated to a host
+running a Xenstore implementation with less features than the one the guest
+is created on.
+
+The visible features are specified via a binary or of the following
+values:
+
+=over 4
+
+=item B<0x00000001>
+
+Xenstore is capable to reconnect to a guest.
+
+=item B<0x00000002>
+
+Xenstore will present an error value in case it disconnects due to an error
+condition.
+
+=back
+
+The features supported by the running Xenstore instance can be retrieved
+via the B<xl info> command in dom0.
+
+The default value is B<0xffffffff>, meaning that all possible Xenstore
+features are visible by the guest.
+
+=back
+
 =head2 Devices
 
 The following options define the paravirtual, emulated and physical
diff --git a/tools/golang/xenlight/helpers.gen.go 
b/tools/golang/xenlight/helpers.gen.go
index b43aad7d00..667030cbd7 100644
--- a/tools/golang/xenlight/helpers.gen.go
+++ b/tools/golang/xenlight/helpers.gen.go
@@ -1175,6 +1175,7 @@ return fmt.Errorf("converting field Vpmu: %v", err)
 if err := x.TrapUnmappedAccesses.fromC(&xc.trap_unmapped_accesses);err != nil {
 return fmt.Errorf("converting field TrapUnmappedAccesses: %v", err)
 }
+x.XenstoreFeatureMask = uint32(xc.xenstore_feature_mask)
 
  return nil}
 
@@ -1710,6 +1711,7 @@ return fmt.Errorf("converting field Vpmu: %v", err)
 if err := x.TrapUnmappedAccesses.toC(&xc.trap_unmapped_accesses); err != nil {
 return fmt.Errorf("converting field TrapUnmappedAccesses: %v", err)
 }
+xc.xenstore_feature_mask = C.uint32_t(x.XenstoreFeatureMask)
 
  return nil
  }
diff --git a/tools/golang/xenlight/types.gen.go 
b/tools/golang/xenlight/types.gen.go
index 4777f528b5..e26b3cdfc7 100644
--- a/tools/golang/xenlight/types.gen.go
+++ b/tools/golang/xenlight/types.gen.go
@@ -608,6 +608,7 @@ Altp2MCount uint32
 VmtraceBufKb int
 Vpmu Defbool
 TrapUnmappedAccesses Defbool
+XenstoreFeatureMask uint32
 }
 
 type DomainBuildInfoTypeUnion interface {
diff --git a/tools/include/libxl.h b/tools/include/libxl.h
index d6b6e5d2dd..185f74d8a8 100644
--- a/tools/include/libxl.h
+++ b/tools/include/libxl.h
@@ -654,6 +654,12 @@
  */
 #define LIBXL_HAVE_DT_OVERLAY_DOMAIN 1
 
+/*
+ * LIBXL_HAVE_XENSTORE_FEATURE_MASK indicates the presence of
+ * xenstore_feature_mask in struct libxl_domain_build_info.
+ */
+#define LIBXL_HAVE_XENSTORE_FEATURE_MASK 1
+
 /*
  * libxl memory management
  *
diff --git a/tools/libs/light/libxl_dom.c b/tools/libs/light/libxl_dom.c
index a61085ca3b..05ebc69534 100644
--- a/tools/libs/light/libxl_dom.c
+++ b/tools/libs/light/libxl_dom.c
@@ -494,9 +494,24 @@ retry_transaction:
     if (!xs_transaction_end(ctx->xsh, t, 0))
         if (errno == EAGAIN)
             goto retry_transaction;
+
+    if (info->xenstore_feature_mask != ~0U) {
+        unsigned int features;
+
+        if (xs_get_features_supported(ctx->xsh, &features) &&
+            !xs_set_features_domain(ctx->xsh, domid,
+                                    features & info->xenstore_feature_mask)) {
+            LOGED(ERROR, domid, "Failed to set Xenstore features");
+            rc = ERROR_FAIL;
+            goto out;
+        }
+    }
+
     xs_introduce_domain(ctx->xsh, domid, state->store_mfn, state->store_port);
+
+ out:
     free(vm_path);
-    return 0;
+    return rc;
 }
 
 static int set_vnuma_info(libxl__gc *gc, uint32_t domid,
diff --git a/tools/libs/light/libxl_types.idl b/tools/libs/light/libxl_types.idl
index fe251649f3..a6030a2dbd 100644
--- a/tools/libs/light/libxl_types.idl
+++ b/tools/libs/light/libxl_types.idl
@@ -739,6 +739,7 @@ libxl_domain_build_info = Struct("domain_build_info",[
 
     ("vpmu", libxl_defbool),
     ("trap_unmapped_accesses", libxl_defbool),
+    ("xenstore_feature_mask", uint32, {'init_val': '~0U'}),
 
     ], dir=DIR_IN,
        copy_deprecated_fn="libxl__domain_build_info_copy_deprecated",
diff --git a/tools/xl/xl_parse.c b/tools/xl/xl_parse.c
index 28cdbf07c2..90c9386f5b 100644
--- a/tools/xl/xl_parse.c
+++ b/tools/xl/xl_parse.c
@@ -1409,6 +1409,9 @@ void parse_config_data(const char *config_source,
     if (!xlu_cfg_get_string (config, "pool", &buf, 0))
         xlu_cfg_replace_string(config, "pool", &c_info->pool_name, 0);
 
+    if (!xlu_cfg_get_long (config, "xenstore_feature_mask", &l, 0))
+        b_info->xenstore_feature_mask = l;
+
     libxl_domain_build_info_init_type(b_info, c_info->type);
 
     if (b_info->type == LIBXL_DOMAIN_TYPE_PVH) {
-- 
2.43.0