init-dom0less: allocate xenstore page if not already allocated

We check if the xenstore page is already allocated. If yes, there is
nothing to do. If no, we proceed allocating it to support old unpatched
Linux kernels.

Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>

tools/helpers/init-dom0less.c

@@ -16,8 +16,34 @@
 
 #include "init-dom-json.h"
 
+#define XENSTORE_PFN_OFFSET 1
 #define STR_MAX_LENGTH 128
 
+static int alloc_xs_page(struct xc_interface_core *xch,
+                         libxl_dominfo *info,
+                         uint64_t *xenstore_pfn)
+{
+    int rc;
+    const xen_pfn_t base = GUEST_MAGIC_BASE >> XC_PAGE_SHIFT;
+    xen_pfn_t p2m = base + XENSTORE_PFN_OFFSET;
+
+    rc = xc_domain_setmaxmem(xch, info->domid,
+                             info->max_memkb + (XC_PAGE_SIZE/1024));
+    if (rc < 0)
+        return rc;
+
+    rc = xc_domain_populate_physmap_exact(xch, info->domid, 1, 0, 0, &p2m);
+    if (rc < 0)
+        return rc;
+
+    *xenstore_pfn = base + XENSTORE_PFN_OFFSET;
+    rc = xc_clear_domain_page(xch, info->domid, *xenstore_pfn);
+    if (rc < 0)
+        return rc;
+
+    return 0;
+}
+
 static int get_xs_page(struct xc_interface_core *xch, libxl_dominfo *info,
                        uint64_t *xenstore_pfn)
 {
@@ -233,9 +259,33 @@ static int init_domain(struct xs_handle *xsh,
         return 0;
 
     /* Get xenstore page */
-    if (get_xs_page(xch, info, &xenstore_pfn) != 0) {
-        printf("Error on getting xenstore page\n");
+    if (get_xs_page(xch, info, &xenstore_pfn) != 0)
         return 1;
+
+    if (xenstore_pfn == ~0ULL) {
+        struct xenstore_domain_interface *intf;
+
+        rc = alloc_xs_page(xch, info, &xenstore_pfn);
+        if (rc != 0) {
+            printf("Error on getting xenstore page\n");
+            return 1;
+        }
+
+        intf = xenforeignmemory_map(xfh, info->domid, PROT_READ | PROT_WRITE, 1,
+                                    &xenstore_pfn, NULL);
+        if (!intf) {
+            printf("Error mapping xenstore page\n");
+            return 1;
+        }
+
+        intf->connection = XENSTORE_RECONNECT;
+        xenforeignmemory_unmap(xfh, intf, 1);
+
+        /* Now everything is ready: set HVM_PARAM_STORE_PFN */
+        rc = xc_hvm_param_set(xch, info->domid, HVM_PARAM_STORE_PFN,
+                xenstore_pfn);
+        if (rc < 0)
+            return rc;
     }
 
     rc = xc_dom_gnttab_seed(xch, info->domid, true,