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[Qemu-devel] [PATCH v2 12/12] block/dmg: improve zeroes handling
From: |
Peter Wu |
Subject: |
[Qemu-devel] [PATCH v2 12/12] block/dmg: improve zeroes handling |
Date: |
Tue, 6 Jan 2015 18:48:15 +0100 |
Disk images may contain large all-zeroes gaps (1.66k sectors or 812 MiB
is seen in the real world). These blocks (type 2) do not need to be
extracted into a temporary buffer, there is no need to allocate memory
for these blocks nor to check its length.
(For the test image, the maximum uncompressed size is 1054371 bytes,
probably for a bzip2-compressed block.)
Signed-off-by: Peter Wu <address@hidden>
---
v2: no changes (did not receive comments last time)
---
block/dmg.c | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)
diff --git a/block/dmg.c b/block/dmg.c
index 8239221..4e24076 100644
--- a/block/dmg.c
+++ b/block/dmg.c
@@ -137,7 +137,9 @@ static void update_max_chunk_size(BDRVDMGState *s, uint32_t
chunk,
uncompressed_sectors = (s->lengths[chunk] + 511) / 512;
break;
case 2: /* zero */
- uncompressed_sectors = s->sectorcounts[chunk];
+ /* as the all-zeroes block may be large, it is treated specially: the
+ * sector is not copied from a large buffer, a simple memset is used
+ * instead. Therefore uncompressed_sectors does not need to be set. */
break;
}
@@ -269,7 +271,9 @@ static int dmg_read_mish_block(BDRVDMGState *s,
DmgHeaderState *ds,
/* sector count */
s->sectorcounts[i] = buff_read_uint64(buffer, offset + 0x10);
- if (s->sectorcounts[i] > DMG_SECTORCOUNTS_MAX) {
+ /* all-zeroes sector (type 2) does not need to be "uncompressed" and
can
+ * therefore be unbounded. */
+ if (s->types[i] != 2 && s->sectorcounts[i] > DMG_SECTORCOUNTS_MAX) {
error_report("sector count %" PRIu64 " for chunk %" PRIu32
" is larger than max (%u)",
s->sectorcounts[i], i, DMG_SECTORCOUNTS_MAX);
@@ -644,7 +648,8 @@ static inline int dmg_read_chunk(BlockDriverState *bs,
uint64_t sector_num)
}
break;
case 2: /* zero */
- memset(s->uncompressed_chunk, 0, 512 * s->sectorcounts[chunk]);
+ /* see dmg_read, it is treated specially. No buffer needs to be
+ * pre-filled, the zeroes can be set directly. */
break;
}
s->current_chunk = chunk;
@@ -663,6 +668,13 @@ static int dmg_read(BlockDriverState *bs, int64_t
sector_num,
if (dmg_read_chunk(bs, sector_num + i) != 0) {
return -1;
}
+ /* Special case: current chunk is all zeroes. Do not perform a memcpy
as
+ * s->uncompressed_chunk may be too small to cover the large all-zeroes
+ * section. dmg_read_chunk is called to find s->current_chunk */
+ if (s->types[s->current_chunk] == 2) { /* all zeroes block entry */
+ memset(buf + i * 512, 0, 512);
+ continue;
+ }
sector_offset_in_chunk = sector_num + i - s->sectors[s->current_chunk];
memcpy(buf + i * 512,
s->uncompressed_chunk + sector_offset_in_chunk * 512, 512);
--
2.2.1
- Re: [Qemu-devel] [PATCH v2 09/12] block/dmg: use SectorNumber from BLKX header, (continued)
- [Qemu-devel] [PATCH v2 02/12] block/dmg: extract mish block decoding functionality, Peter Wu, 2015/01/06
- [Qemu-devel] [PATCH v2 06/12] block/dmg: process XML plists, Peter Wu, 2015/01/06
- [Qemu-devel] [PATCH v2 12/12] block/dmg: improve zeroes handling,
Peter Wu <=
- [Qemu-devel] [PATCH v2 10/12] block/dmg: factor out block type check, Peter Wu, 2015/01/06
- [Qemu-devel] [PATCH v2 11/12] block/dmg: support bzip2 block entry types, Peter Wu, 2015/01/06
- Re: [Qemu-devel] [PATCH v2 00/12] block/dmg: (compatibility) fixes and bzip2 support, Stefan Hajnoczi, 2015/01/14