[PATCH v5 8/9] xen/arm: introduce legacy dom0less option for xenstore allocation

[Stefano Stabellini <stefano.stabellini@xxxxxxx>]

The new xenstore page allocation scheme might break older unpatches
Linux kernels that do not check for the Xenstore connection status
before proceeding with Xenstore initialization.

Introduce a dom0less configuration option to retain the older behavior,
which is not compatible with 1:1 mapped guests, but it will work with
older legacy kernel versions.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>
---
 docs/misc/arm/device-tree/booting.txt |  5 +++++
 xen/arch/arm/dom0less-build.c         | 13 ++++++++++++-
 xen/arch/arm/include/asm/kernel.h     | 14 +++++++++++---
 3 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/docs/misc/arm/device-tree/booting.txt 
b/docs/misc/arm/device-tree/booting.txt
index ff70d44462..8fa3da95be 100644
--- a/docs/misc/arm/device-tree/booting.txt
+++ b/docs/misc/arm/device-tree/booting.txt
@@ -222,6 +222,11 @@ with the following properties:
     Xen PV interfaces, including grant-table and xenstore, will be
     enabled for the VM.
 
+    - "legacy"
+    Same as above, but the way the xenstore page is allocated is not
+    compatible with 1:1 mapped guests. On the other hand, it works with
+    older Linux kernels.
+
     - "disabled"
     Xen PV interfaces are disabled.
 
diff --git a/xen/arch/arm/dom0less-build.c b/xen/arch/arm/dom0less-build.c
index 046439eb87..9afdbca8b8 100644
--- a/xen/arch/arm/dom0less-build.c
+++ b/xen/arch/arm/dom0less-build.c
@@ -799,6 +799,13 @@ static int __init construct_domU(struct domain *d,
         else
             panic("At the moment, Xenstore support requires dom0 to be 
present\n");
     }
+    else if ( rc == 0 && !strcmp(dom0less_enhanced, "legacy") )
+    {
+        if ( hardware_domain )
+            kinfo.dom0less_feature = DOM0LESS_ENHANCED_LEGACY;
+        else
+            panic("At the moment, Xenstore support requires dom0 to be 
present\n");
+    }
     else if ( rc == 0 && !strcmp(dom0less_enhanced, "no-xenstore") )
         kinfo.dom0less_feature = DOM0LESS_ENHANCED_NO_XS;
 
@@ -848,13 +855,17 @@ static int __init construct_domU(struct domain *d,
     if ( rc < 0 )
         return rc;
 
-    if ( kinfo.dom0less_feature & DOM0LESS_XENSTORE )
+    if ( kinfo.dom0less_feature & (DOM0LESS_XENSTORE|DOM0LESS_XS_LEGACY) )
     {
         ASSERT(hardware_domain);
         rc = alloc_xenstore_evtchn(d);
         if ( rc < 0 )
             return rc;
+        d->arch.hvm.params[HVM_PARAM_STORE_PFN] = ~0ULL;
+    }
 
+    if ( kinfo.dom0less_feature & DOM0LESS_XENSTORE )
+    {
         rc = alloc_xenstore_page(d);
         if ( rc < 0 )
             return rc;
diff --git a/xen/arch/arm/include/asm/kernel.h 
b/xen/arch/arm/include/asm/kernel.h
index de3f945ae5..4c2ae0b32b 100644
--- a/xen/arch/arm/include/asm/kernel.h
+++ b/xen/arch/arm/include/asm/kernel.h
@@ -17,16 +17,24 @@
  *                          default features (excluding Xenstore) will be
  *                          available. Note that an OS *must* not rely on the
  *                          availability of Xen features if this is not set.
- * DOM0LESS_XENSTORE:       Xenstore will be enabled for the VM. This feature
- *                          can't be enabled without the
- *                          DOM0LESS_ENHANCED_NO_XS.
+ * DOM0LESS_XENSTORE:       Xenstore will be enabled for the VM. The
+ *                          xenstore page allocation is done by Xen at
+ *                          domain creation. This feature can't be
+ *                          enabled without the DOM0LESS_ENHANCED_NO_XS.
+ * DOM0LESS_XS_LEGACY       Xenstore will be enabled for the VM, the
+ *                          xenstore page allocation will happen in
+ *                          init-dom0less. This feature can't be enabled
+ *                          without the DOM0LESS_ENHANCED_NO_XS.
  * DOM0LESS_ENHANCED:       Notify the OS it is running on top of Xen. All the
  *                          default features (including Xenstore) will be
  *                          available. Note that an OS *must* not rely on the
  *                          availability of Xen features if this is not set.
+ * DOM0LESS_ENHANCED_LEGACY:Same as before, but using DOM0LESS_XS_LEGACY.
  */
 #define DOM0LESS_ENHANCED_NO_XS  BIT(0, U)
 #define DOM0LESS_XENSTORE        BIT(1, U)
+#define DOM0LESS_XS_LEGACY       BIT(2, U)
+#define DOM0LESS_ENHANCED_LEGACY (DOM0LESS_ENHANCED_NO_XS | DOM0LESS_XS_LEGACY)
 #define DOM0LESS_ENHANCED        (DOM0LESS_ENHANCED_NO_XS | DOM0LESS_XENSTORE)
 
 struct kernel_info {
-- 
2.25.1