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| author | Jan Beulich <jbeulich@suse.com> | 2012-09-03 08:17:50 +0200 |
|---|---|---|
| committer | Jan Beulich <jbeulich@suse.com> | 2012-09-03 08:17:50 +0200 |
| commit | aa1c1f4c57720d3f8426aab89cf8217dd687a651 (patch) | |
| tree | 969de4287a76a9d956f7607bb6cc48bdc2814b2b | |
| parent | 5fdfc1eedd92bc5358ee3b6e63efb0d60fb835c4 (diff) | |
| download | xen-aa1c1f4c57720d3f8426aab89cf8217dd687a651.tar.gz xen-aa1c1f4c57720d3f8426aab89cf8217dd687a651.tar.bz2 xen-aa1c1f4c57720d3f8426aab89cf8217dd687a651.zip | |
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@citrix.com>
Suggested-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>
| -rw-r--r-- | xen/include/asm-x86/mm.h | 6 |
1 files changed, 6 insertions, 0 deletions
diff --git a/xen/include/asm-x86/mm.h b/xen/include/asm-x86/mm.h index 4cba2765fb..2ed53d9416 100644 --- a/xen/include/asm-x86/mm.h +++ b/xen/include/asm-x86/mm.h @@ -323,6 +323,12 @@ static inline struct page_info *__virt_to_page(const void *v) 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))) * |