[Xen-devel] [PATCH v5] x86/mm: Clean up p2m_finish_type_change return value

[Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>]

In the case of any errors, finish_type_change() passes values returned
from p2m->recalc() up the stack (with some exceptions in the case where
an error is expected); this eventually ends up being returned to the
XEN_DOMOP_map_mem_type_to_ioreq_server hypercall.

However, on Intel processors (but not on AMD processor), p2m->recalc()
can also return '1' as well as '0'.  This case is handled very
inconsistently: finish_type_change() will return the value of the final
entry it attempts, discarding results for other entries;
p2m_finish_type_change() will attempt to accumulate '1's, so that it
returns '1' if any of the calls to finish_type_change() returns '1'; and
dm_op() will again return '1' only if the very last call to
p2m_finish_type_change() returns '1'.  The result is that the
XEN_DMOP_map_mem_type_to_ioreq_server() hypercall will sometimes return
0 and sometimes return 1 on success, in an unpredictable manner.

The hypercall documentation doesn't mention return values; but it's not
clear what the caller could do with the information about whether
entries had been changed or not.  At the moment it's always 0 on AMD
boxes, and *usually* 1 on Intel boxes; so nothing can be relying on a
'1' return value for correctness (or if it is, it's broken).

Make the return value on success consistently '0' by only returning
0/-ERROR from finish_type_change().  Also remove the accumulation code
from p2m_finish_type_change().

Suggested-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
Signed-off-by: Alexandru Isaila <aisaila@xxxxxxxxxxxxxxx>

---
Changes since V4:
        - Undo changes in p2m->recalc() and have 1 turn into 0 in
        finish_type_change().
---
 xen/arch/x86/mm/p2m.c | 16 +++++-----------
 1 file changed, 5 insertions(+), 11 deletions(-)

diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
index b9bbb8f485..5508cedc55 100644
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -1158,7 +1158,7 @@ void p2m_change_type_range(struct domain *d,
  * Finish p2m type change for gfns which are marked as need_recalc in a range.
  * Uses the current p2m's max_mapped_pfn to further clip the invalidation
  * range for alternate p2ms.
- * Returns: 0/1 for success, negative for failure
+ * Returns: 0 for success, negative for failure
  */
 static int finish_type_change(struct p2m_domain *p2m,
                               gfn_t first_gfn, unsigned long max_nr)
@@ -1174,9 +1174,9 @@ static int finish_type_change(struct p2m_domain *p2m,
         /*
          * ept->recalc could return 0/1/-ENOMEM. pt->recalc could return
          * 0/-ENOMEM/-ENOENT, -ENOENT isn't an error as we are looping
-         * gfn here.
+         * gfn here. If rc is 1 we need to have it 0 for success.
          */
-        if ( rc == -ENOENT )
+        if ( rc == -ENOENT || rc == 1 )
             rc = 0;
         else if ( rc < 0 )
         {
@@ -1213,19 +1213,13 @@ int p2m_finish_type_change(struct domain *d,
             if ( d->arch.altp2m_eptp[i] != mfn_x(INVALID_MFN) )
             {
                 struct p2m_domain *altp2m = d->arch.altp2m_p2m[i];
-                int rc1;
 
                 p2m_lock(altp2m);
-                rc1 = finish_type_change(altp2m, first_gfn, max_nr);
+                rc = finish_type_change(altp2m, first_gfn, max_nr);
                 p2m_unlock(altp2m);
 
-                if ( rc1 < 0 )
-                {
-                    rc = rc1;
+                if ( rc < 0 )
                     goto out;
-                }
-
-                rc |= rc1;
             }
     }
 #endif
-- 
2.17.1


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