[PATCH] Argo: drop meaningless mfn_valid() check

[Jan Beulich <jbeulich@xxxxxxxx>]

Holding a valid struct page_info * in hands already means the referenced
MFN is valid; there's no need to check that again. Convert the checking
logic to a switch(), to help keeping the extra (and questionable) x86-
only check in somewhat tidy shape.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Initially I had this (with less code churn) as

#ifdef CONFIG_X86
    if ( p2mt == p2m_ram_logdirty )
        ret = -EAGAIN;
    else
#endif
    if ( (p2mt != p2m_ram_rw) ||
         !get_page_type(page, PGT_writable_page) )
        ret = -EINVAL;

But the "else" placement seemed too ugly to me. Otoh there better
wouldn't be any special casing of log-dirty here (and instead such a
page be converted, perhaps right in check_get_page_from_gfn() when
readonly=false), at which point the odd "else" would go away, and the
if() likely again be preferable over the switch().

--- a/xen/common/argo.c
+++ b/xen/common/argo.c
@@ -1421,15 +1421,24 @@ find_ring_mfn(struct domain *d, gfn_t gf
         return ret;
 
     *mfn = page_to_mfn(page);
-    if ( !mfn_valid(*mfn) )
-        ret = -EINVAL;
+
+    switch ( p2mt )
+    {
+    case p2m_ram_rw:
+        if ( !get_page_and_type(page, d, PGT_writable_page) )
+            ret = -EINVAL;
+        break;
+
 #ifdef CONFIG_X86
-    else if ( p2mt == p2m_ram_logdirty )
+    case p2m_ram_logdirty:
         ret = -EAGAIN;
+        break;
 #endif
-    else if ( (p2mt != p2m_ram_rw) ||
-              !get_page_and_type(page, d, PGT_writable_page) )
+
+    default:
         ret = -EINVAL;
+        break;
+    }
 
     put_page(page);