[Xen-changelog] [xen-unstable] x86: comment opaque expression in __page_to_virt()

[Xen patchbot-unstable <patchbot@xxxxxxx>]

# HG changeset patch
# User Jan Beulich <jbeulich@xxxxxxxx>
# Date 1346653070 -7200
# Node ID 8fbdcc1c5613dc55c23be8b56f52a37f4d89e9fa
# Parent  34e22386e7127f3ef77ac73098fc50c0fe7bab1f
x86: comment opaque expression in __page_to_virt()

mm.h's __page_to_virt() has a rather opaque expression. Comment it.

Reported-By: Ian Campbell <ian.campbell@xxxxxxxxxx>
Suggested-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Acked-by: Keir Fraser <keir@xxxxxxx>
---


diff -r 34e22386e712 -r 8fbdcc1c5613 xen/include/asm-x86/mm.h
--- a/xen/include/asm-x86/mm.h  Fri Aug 31 21:15:31 2012 +0100
+++ b/xen/include/asm-x86/mm.h  Mon Sep 03 08:17:50 2012 +0200
@@ -323,6 +323,12 @@ static inline struct page_info *__virt_t
 static inline void *__page_to_virt(const struct page_info *pg)
 {
     ASSERT((unsigned long)pg - FRAMETABLE_VIRT_START < FRAMETABLE_VIRT_END);
+    /*
+     * (sizeof(*pg) & -sizeof(*pg)) selects the LS bit of sizeof(*pg). The
+     * division and re-multiplication avoids one shift when sizeof(*pg) is a
+     * power of two (otherwise there would be a right shift followed by a
+     * left shift, which the compiler can't know it can fold into one).
+     */
     return (void *)(DIRECTMAP_VIRT_START +
                     ((unsigned long)pg - FRAMETABLE_VIRT_START) /
                     (sizeof(*pg) / (sizeof(*pg) & -sizeof(*pg))) *

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