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Re: [Qemu-devel] [PATCH RFC v2 8/8] migration: add migration/dirty-bitma

From: John Snow
Subject: Re: [Qemu-devel] [PATCH RFC v2 8/8] migration: add migration/dirty-bitmap.c
Date: Fri, 13 Feb 2015 12:32:48 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0

On 02/13/2015 04:06 AM, Vladimir Sementsov-Ogievskiy wrote:

+    blk_mig_reset_dirty_cursor();
+    dirty_phase(f, false);
+    QSIMPLEQ_FOREACH(dbms, &dirty_bitmap_mig_state.dbms_list, entry) {
+        uint8_t flags = DIRTY_BITMAP_MIG_FLAG_DEVICE_NAME |
+                        DIRTY_BITMAP_MIG_FLAG_BITMAP_NAME |
+                        DIRTY_BITMAP_MIG_FLAG_ENABLED;
+        qemu_put_byte(f, flags);
+        qemu_put_name(f, bdrv_get_device_name(dbms->bs));
+        qemu_put_name(f, bdrv_dirty_bitmap_name(dbms->bitmap));
+        qemu_put_byte(f, bdrv_dirty_bitmap_enabled(dbms->bitmap));
+    }
+    qemu_put_byte(f, DIRTY_BITMAP_MIG_FLAG_EOS);
+    DPRINTF("Dirty bitmaps migration completed\n");
+    dirty_bitmap_mig_cleanup();
+    return 0;

I suppose we don't need a flag that distinctly SAYS this is the end
section, since we can tell by omission of
Hmm. I think it simplifies the logic (to use EOS after each section).
And the same approach is in migration/block.c.. It's a question about
which format is better:  "Each section for dirty_bitmap_load ends with
EOS" or "Each section for dirty_bitmap_load ends with EOS except the
last one. The last one may be recognized by absent NORMAL_CHUNK and

Oh, sorry, no, it's important EOS. There are several blocks with no
*_CHUNK! Several bitmaps. And loop in dirty_bitmap_load will read them
iteratively, and it will finish when find EOS.

Sorry, I worded that poorly. I was wondering why you didn't have an explicit "end of bitmap" flag, and I realized that you are doing this check essentially by the absence of the NORMAL_CHUNK/ZERO_CHUNK flags.

This is really just a comment on my part; I was expecting a more distinct "It is now safe to rebuild the bitmap" flag and was just commenting on why we didn't necessarily need one.

I think in another comment I point out that an "end of bitmap" flag might make the loading function simpler, and I still think that might be nice.

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