[Xen-devel] [PATCH] Re: [RFC PATCH] xen: comment opaque expression in __page_to_virt

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 31.08.12 at 16:36, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx> wrote:
> Jan Beulich writes ("Re: [RFC PATCH] xen: comment opaque expression in 
> __page_to_virt"):
>> No, that's not precise. There's really not much of a win to be had
>> on 32-bit (division by 3 and division by 24 (sizeof(struct page_info))
>> should be the same in speed.
>> 
>> The win is on x86-64, where sizeof(struct page_info) is a power
>> of 2, and hence the pair of shifts (right, then left) can be reduced
>> to a single one.
>> 
>> Yet (for obvious reasons) the code ought to not break anything
>> if even on x86-64 the size of the structure would change, hence
>> it needs to be that complex (and can't be broken into separate,
>> simpler implementations for 32- and 64-bits).
> 
> Thanks.  Do you want to post a revised version of my patch or shall I
> do so ?  (If so please confirm that I should put your s-o-b on it for
> your wording above.)

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>

--- 2012-08-08.orig/xen/include/asm-x86/mm.h    2012-06-20 17:34:02.000000000 
+0200
+++ 2012-08-08/xen/include/asm-x86/mm.h 2012-08-31 16:50:50.000000000 +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))) *




_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel