Re: [Xen-devel] The two image formats called qcow

[Kevin Wolf <kwolf@xxxxxxx>]

Otavio Salvador schrieb:
> Kevin Wolf <kwolf@xxxxxxx> writes:
> 
>> Is it an option to convert broken images to big endian when it is opened
>> for the first time in ioemu? In this case, the fix for older versions
>> could be in one place at least instead of being scattered over the whole
>> file. Then you wouldn't be able to open such a file with tapdisk again,
>> though.
> 
> If it could be done, a note in release notes would be required to warn
> users that when it has been opened in a newer version using ioemu it
> wouldn't be possible to be used again by tap driver anymore.

I agree, a note wouldn't be wrong.

> But yes, this would be an easy to use option if it's not too messy and
> takes too long to convert the image (otherwise user can think that the
> system has freeze).

Converting the image means byte-swapping the L1 table which is some
kilobytes in size. This is done in no time, the user won't even notice
the delay. The attached patch implements this conversion.

What should we do with the tapdisk implementation? Leave it broken and
hope that it will disappear soon, add support for big endian L1 tables
or do a conversion the other way round? The latter doesn't feel right
(in fact it would be intentionally breaking a correct image), but adding
support for big endian is much more critical because we end up with
"mixed endian" if we miss one conversion...

Kevin


ioemu: Fix L1 table endianess of qcow images created by tapdisk

The qemu/ioemu implementation of the qcow format uses a big endian L1
table. tapdisk omits the necessary conversion, so qcow images have the
wrong endianess and cannot be read by correct implementations of qcow.

This patch detects broken tapdisk images and converts their L1 tables to
big endian when the image file is opened in ioemu for the first time.
The fixed image has a new flag EXTHDR_L1_BIG_ENDIAN set in the extended
header.

Note that a converted image cannot be opened by tapdisk again.

Signed-off-by: Kevin Wolf <kwolf@xxxxxxx>

diff -r 81147306041a tools/ioemu/block-qcow.c
--- a/tools/ioemu/block-qcow.c  Tue Mar 25 14:37:43 2008
+++ b/tools/ioemu/block-qcow.c  Wed Mar 26 15:25:37 2008
@@ -37,6 +37,11 @@
 
 #define QCOW_OFLAG_COMPRESSED (1LL << 63)
 
+#define XEN_MAGIC  (('X' << 24) | ('E' << 16) | ('N' << 8) | 0xfb)
+
+#define EXTHDR_SPARSE_FILE      0x01
+#define EXTHDR_L1_BIG_ENDIAN    0x02
+
 typedef struct QCowHeader {
     uint32_t magic;
     uint32_t version;
@@ -49,6 +54,14 @@
     uint32_t crypt_method;
     uint64_t l1_table_offset;
 } QCowHeader;
+
+/*Extended header for Xen enhancements*/
+typedef struct QCowHeader_ext {
+    uint32_t xmagic;
+    uint32_t cksum;
+    uint32_t min_cluster_alloc;
+    uint32_t flags;
+} QCowHeader_ext;
 
 #define L2_CACHE_SIZE 16
 
@@ -137,6 +150,51 @@
     if (bdrv_pread(s->hd, s->l1_table_offset, s->l1_table, s->l1_size * 
sizeof(uint64_t)) != 
         s->l1_size * sizeof(uint64_t))
         goto fail;
+
+    /* Try to detect old tapdisk images. They have to be fixed because they
+     * don't use big endian but native endianess for the L1 table */
+       if (header.backing_file_offset == 0 && s->l1_table_offset % 4096 == 0) {
+       
+        QCowHeader_ext exthdr;
+        uint64_t l1_bytes = s->l1_size * sizeof(uint64_t);
+
+        if (bdrv_pread(s->hd, sizeof(header), &exthdr, sizeof(exthdr)) 
+                != sizeof(exthdr))
+            goto end_xenhdr;
+
+               be32_to_cpus(&exthdr.xmagic);
+               if (exthdr.xmagic != XEN_MAGIC) 
+                       goto end_xenhdr;
+
+               be32_to_cpus(&exthdr.flags);
+        if (exthdr.flags & EXTHDR_L1_BIG_ENDIAN)
+            goto end_xenhdr;
+
+        /* The image is broken. Fix it. */
+        fprintf(stderr, "qcow: Converting image to big endian L1 table\n");
+
+        for(i = 0;i < s->l1_size; i++) {
+            cpu_to_be64s(&s->l1_table[i]);
+        }
+    
+        if (bdrv_pwrite(s->hd, s->l1_table_offset, s->l1_table, 
+                l1_bytes) != l1_bytes) {
+            fprintf(stderr, "qcow: Failed to write new L1 table\n");
+            goto fail;
+        }
+
+        exthdr.flags |= EXTHDR_L1_BIG_ENDIAN;
+        cpu_to_be32s(&exthdr.flags);
+        
+        if (bdrv_pwrite(s->hd, sizeof(header), &exthdr, sizeof(exthdr)) 
+                != sizeof(exthdr)) {
+            fprintf(stderr, "qcow: Failed to write extended header\n");
+            goto fail;
+        }
+    }
+end_xenhdr:
+
+    /* L1 table is big endian now */
     for(i = 0;i < s->l1_size; i++) {
         be64_to_cpus(&s->l1_table[i]);
     }

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel