Patchwork [PULL,29/36] block: vhdx - remove BAT file offset bit shifting

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Submitter Stefan Hajnoczi
Date Nov. 7, 2013, 1:12 p.m.
Message ID <1383829964-32364-30-git-send-email-stefanha@redhat.com>
Download mbox | patch
Permalink /patch/289349/
State New
Headers show

Comments

Stefan Hajnoczi - Nov. 7, 2013, 1:12 p.m.
From: Jeff Cody <jcody@redhat.com>

Bit shifting can be fun, but in this case it was unnecessary.  The
upper 44 bits of the 64-bit BAT entry is specifies the File Offset,
so we shifted the bits to get access to the value.

However, per the spec the value is in MB.  So we dutifully shifted back
to the left by 20 bits, to convert to a true uint64_t file offset.

This replaces those steps with just a bit mask, to get rid of the lower
20 bits instead.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 block/vhdx.c | 6 ++----
 block/vhdx.h | 1 -
 2 files changed, 2 insertions(+), 5 deletions(-)

Patch

diff --git a/block/vhdx.c b/block/vhdx.c
index 050f071..158edab 100644
--- a/block/vhdx.c
+++ b/block/vhdx.c
@@ -985,7 +985,7 @@  static void vhdx_block_translate(BDRVVHDXState *s, int64_t sector_num,
 
     sinfo->bytes_avail = sinfo->sectors_avail << s->logical_sector_size_bits;
 
-    sinfo->file_offset = s->bat[sinfo->bat_idx] >> VHDX_BAT_FILE_OFF_BITS;
+    sinfo->file_offset = s->bat[sinfo->bat_idx] & VHDX_BAT_FILE_OFF_MASK;
 
     sinfo->block_offset = block_offset << s->logical_sector_size_bits;
 
@@ -999,7 +999,6 @@  static void vhdx_block_translate(BDRVVHDXState *s, int64_t sector_num,
      * in the block, and add in the payload data block offset
      * in the file, in bytes, to get the final read address */
 
-    sinfo->file_offset <<= 20;  /* now in bytes, rather than 1MB units */
     sinfo->file_offset += sinfo->block_offset;
 }
 
@@ -1098,8 +1097,7 @@  static void vhdx_update_bat_table_entry(BlockDriverState *bs, BDRVVHDXState *s,
 {
     /* The BAT entry is a uint64, with 44 bits for the file offset in units of
      * 1MB, and 3 bits for the block state. */
-    s->bat[sinfo->bat_idx]  = ((sinfo->file_offset>>20) <<
-                               VHDX_BAT_FILE_OFF_BITS);
+    s->bat[sinfo->bat_idx]  = sinfo->file_offset;
 
     s->bat[sinfo->bat_idx] |= state & VHDX_BAT_STATE_BIT_MASK;
 
diff --git a/block/vhdx.h b/block/vhdx.h
index f331548..d3598e0 100644
--- a/block/vhdx.h
+++ b/block/vhdx.h
@@ -229,7 +229,6 @@  typedef struct QEMU_PACKED VHDXLogDataSector {
 /* upper 44 bits are the file offset in 1MB units lower 3 bits are the state
    other bits are reserved */
 #define VHDX_BAT_STATE_BIT_MASK 0x07
-#define VHDX_BAT_FILE_OFF_BITS (64 - 44)
 #define VHDX_BAT_FILE_OFF_MASK  0xFFFFFFFFFFF00000 /* upper 44 bits */
 typedef uint64_t VHDXBatEntry;