Message-ID: <4B74B70A.4030805@lab.ntt.co.jp>
Date: Fri, 12 Feb 2010 11:03:54 +0900
From: OHMURA Kei <ohmura.kei@lab.ntt.co.jp>
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References: <4B728FF9.6010707@lab.ntt.co.jp> <4B72B28E.6010801@redhat.com>
	<4B72D706.3070602@codemonkey.ws>
In-Reply-To: <4B72D706.3070602@codemonkey.ws>
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Subject: [Qemu-devel] Re: [PATCH v2] qemu-kvm: Speed up of the
	dirty-bitmap-traveling
To: Anthony Liguori <anthony@codemonkey.ws>, Avi Kivity <avi@redhat.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, "kvm@vger.kernel.org" <kvm@vger.kernel.org>
Cc: ohmura.kei@lab.ntt.co.jp, mtosatti@redhat.com, drepper@redhat.com, Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>

On 02/11/2010 Anthony Liguori <anthony@codemonkey.ws> wrote:
> Oh, I see what's happening here. Yes, I think a leul_to_cpu() makes more
> sense.

Maybe I'm missing something here.
I couldn't find leul_to_cpu(), so have defined it in bswap.h.
Correct?

--- a/bswap.h
+++ b/bswap.h
@@ -205,8 +205,10 @@ static inline void cpu_to_be32wu(uint32_t *p, uint32_t v)
 
 #ifdef HOST_WORDS_BIGENDIAN
 #define cpu_to_32wu cpu_to_be32wu
+#define leul_to_cpu(v) le ## HOST_LONG_BITS ## _to_cpu(v)
 #else
 #define cpu_to_32wu cpu_to_le32wu
+#define leul_to_cpu(v) (v)
 #endif


On 02/10/2010 Ulrich Drepper <drepper@redhat.com> wrote:
> If you're optimizing this code you might want to do it all.  The
> compiler might not see through the bswap call and create unnecessary
> data dependencies.  Especially problematic if the bitmap is really
> sparse.  Also, the outer test is != while the inner test is >.  Be
> consistent.  I suggest to replace the inner loop with
> 
>      do {
>        ...
>      } while (c != 0);
> 
> Depending on how sparse the bitmap is populated this might reduce the
> number of data dependencies quite a bit.

Combining all comments, the code would be like this.
     
 if (bitmap_ul[i] != 0) {
     c = leul_to_cpu(bitmap_ul[i]);
     do {
         j = ffsl(c) - 1;
         c &= ~(1ul << j);
         page_number = i * HOST_LONG_BITS + j;
         addr1 = page_number * TARGET_PAGE_SIZE;
         addr = offset + addr1;
         ram_addr = cpu_get_physical_page_desc(addr);
         cpu_physical_memory_set_dirty(ram_addr);
     } while (c != 0);
 }