Xen devel - Oct 2009 - Re: [Xen-changelog] [xen-unstable] stubdom: mmap on /dev/mem support

Xen patchbot-unstable, le Wed 21 Oct 2009 08:10:15 -0700, a écrit
:
> +        unsigned long mfns[n];
> +        for (i = 0; i < n; i++)
> +            mfns[i] = ((unsigned long) offset + (i * PAGE_SIZE)) >>
PAGE_SHIFT;
Mmm, is there a reason for adding (i*PAGE_SIZE) before right-shifting?
> +        return map_frames_ex(mfns, n, 1, 0, 1, DOMID_IO, 0,
_PAGE_PRESENT|_PAGE_RW);
Actually I think the whole could just be

	unsigned long first_mfn = offset >> PAGE_SHIFT;
        return map_frames_ex(&first_mfn, n, 0, 1, 1, DOMID_IO, 0,
_PAGE_PRESENT|_PAGE_RW);

which avoids the need to prepare a table.

Samuel

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On Wed, 21 Oct 2009, Samuel Thibault wrote:
> Xen patchbot-unstable, le Wed 21 Oct 2009 08:10:15 -0700, a écrit :
> > +        unsigned long mfns[n];
> > +        for (i = 0; i < n; i++)
> > +            mfns[i] = ((unsigned long) offset + (i * PAGE_SIZE))
>> PAGE_SHIFT;
> 
> Mmm, is there a reason for adding (i*PAGE_SIZE) before right-shifting?
> 
> > +        return map_frames_ex(mfns, n, 1, 0, 1, DOMID_IO, 0,
_PAGE_PRESENT|_PAGE_RW);
> 
> Actually I think the whole could just be
> 
> 	unsigned long first_mfn = offset >> PAGE_SHIFT;
>         return map_frames_ex(&first_mfn, n, 0, 1, 1, DOMID_IO, 0,
_PAGE_PRESENT|_PAGE_RW);
> 
> which avoids the need to prepare a table.
Yeah, you are right.
Both versions are fine for me.

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Optimize mmap(open("/dev/mem"))

Set map_frames_ex''s stride parameter to 0 and increment to 1 to avoid
building an explicit list of mfns.

Signed-Off-By: Samuel Thibault <samuel.thibault@ens-lyon.org>

diff -r 37829fd7c1e3 extras/mini-os/lib/sys.c
--- a/extras/mini-os/lib/sys.c	Wed Oct 21 16:08:28 2009 +0100
+++ b/extras/mini-os/lib/sys.c	Wed Oct 21 18:10:33 2009 +0200
@@ -1256,11 +1268,8 @@
         unsigned long zero = 0;
         return map_frames_ex(&zero, n, 0, 0, 1, DOMID_SELF, 0, 0);
     } else if (files[fd].type == FTYPE_MEM) {
-        int i;
-        unsigned long mfns[n];
-        for (i = 0; i < n; i++)
-            mfns[i] = ((unsigned long) offset + (i * PAGE_SIZE)) >>
PAGE_SHIFT;
-        return map_frames_ex(mfns, n, 1, 0, 1, DOMID_IO, 0,
_PAGE_PRESENT|_PAGE_RW);
+        unsigned long first_mfn = offset >> PAGE_SHIFT;
+        return map_frames_ex(&first_mfn, n, 0, 1, 1, DOMID_IO, 0,
_PAGE_PRESENT|_PAGE_RW);
     } else ASSERT(0);
 }
 

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