[PATCH 2/2] xen/x86: add a way to obtain the needed number of memory map entries

[Juergen Gross <jgross@suse.com>]

Today there is no way for a domain to obtain the number of entries of
the machine memory map returned by XENMEM_machine_memory_map hypercall.

Modify the interface to return just the needed number of map entries
in case the buffer was specified as NULL.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
 xen/arch/x86/mm.c           | 38 +++++++++++++++++++++++---------------
 xen/include/public/memory.h |  2 ++
 2 files changed, 25 insertions(+), 15 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index f8e679d..d384022 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4736,15 +4736,18 @@ static int _handle_iomem_range(unsigned long s, unsigned long e,
         XEN_GUEST_HANDLE_PARAM(e820entry_t) buffer_param;
         XEN_GUEST_HANDLE(e820entry_t) buffer;
 
-        if ( ctxt->n + 1 >= ctxt->map.nr_entries )
-            return -E2BIG;
-        ent.addr = (uint64_t)ctxt->s << PAGE_SHIFT;
-        ent.size = (uint64_t)(s - ctxt->s) << PAGE_SHIFT;
-        ent.type = E820_RESERVED;
-        buffer_param = guest_handle_cast(ctxt->map.buffer, e820entry_t);
-        buffer = guest_handle_from_param(buffer_param, e820entry_t);
-        if ( __copy_to_guest_offset(buffer, ctxt->n, &ent, 1) )
-            return -EFAULT;
+        if ( !guest_handle_is_null(ctxt->map.buffer) )
+        {
+            if ( ctxt->n + 1 >= ctxt->map.nr_entries )
+                return -E2BIG;
+            ent.addr = (uint64_t)ctxt->s << PAGE_SHIFT;
+            ent.size = (uint64_t)(s - ctxt->s) << PAGE_SHIFT;
+            ent.type = E820_RESERVED;
+            buffer_param = guest_handle_cast(ctxt->map.buffer, e820entry_t);
+            buffer = guest_handle_from_param(buffer_param, e820entry_t);
+            if ( __copy_to_guest_offset(buffer, ctxt->n, &ent, 1) )
+                return -EFAULT;
+        }
         ctxt->n++;
     }
     ctxt->s = e + 1;
@@ -4978,6 +4981,7 @@ long arch_memory_op(unsigned long cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
         XEN_GUEST_HANDLE(e820entry_t) buffer;
         XEN_GUEST_HANDLE_PARAM(e820entry_t) buffer_param;
         unsigned int i;
+        bool store;
 
         rc = xsm_machine_memory_map(XSM_PRIV);
         if ( rc )
@@ -4986,9 +4990,10 @@ long arch_memory_op(unsigned long cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
         if ( copy_from_guest(&ctxt.map, arg, 1) )
             return -EFAULT;
 
+        store = !guest_handle_is_null(ctxt.map.buffer);
         buffer_param = guest_handle_cast(ctxt.map.buffer, e820entry_t);
         buffer = guest_handle_from_param(buffer_param, e820entry_t);
-        if ( !guest_handle_okay(buffer, ctxt.map.nr_entries) )
+        if ( store && !guest_handle_okay(buffer, ctxt.map.nr_entries) )
             return -EFAULT;
 
         for ( i = 0, ctxt.n = 0, ctxt.s = 0; i < e820.nr_map; ++i, ++ctxt.n )
@@ -5005,13 +5010,16 @@ long arch_memory_op(unsigned long cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
                 if ( rc )
                     break;
             }
-            if ( ctxt.map.nr_entries <= ctxt.n + 1 )
+            if ( store )
             {
-                rc = -E2BIG;
-                break;
+                if ( ctxt.map.nr_entries <= ctxt.n + 1 )
+                {
+                    rc = -E2BIG;
+                    break;
+                }
+                if ( __copy_to_guest_offset(buffer, ctxt.n, e820.map + i, 1) )
+                    return -EFAULT;
             }
-            if ( __copy_to_guest_offset(buffer, ctxt.n, e820.map + i, 1) )
-                return -EFAULT;
             ctxt.s = PFN_UP(e820.map[i].addr + e820.map[i].size);
         }
 
diff --git a/xen/include/public/memory.h b/xen/include/public/memory.h
index 20df769..2a61e11 100644
--- a/xen/include/public/memory.h
+++ b/xen/include/public/memory.h
@@ -341,6 +341,8 @@ DEFINE_XEN_GUEST_HANDLE(xen_memory_map_t);
  * XENMEM_memory_map.
  * In case of a buffer not capable to hold all entries of the physical
  * memory map -E2BIG is returned and the buffer is filled completely.
+ * Specifying buffer as NULL will return the number of entries required
+ * to store the complete memory map.
  * arg == addr of xen_memory_map_t.
  */
 #define XENMEM_machine_memory_map   10
-- 
2.10.2


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