[Xen-devel] [PATCH v3] 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 V2:
        - Update commit msg.
---
 xen/arch/x86/mm/p2m.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
index b9bbb8f485..bcf8cad423 100644
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -1213,22 +1213,17 @@ 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
+    rc = 0;
 
  out:
     p2m_unlock(hostp2m);
-- 
2.17.1


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