diff -r 6eb1a5d782bf tools/blktap/drivers/block-qcow.c
--- a/tools/blktap/drivers/block-qcow.c	Thu Oct 30 19:48:08 2008 -0700
+++ b/tools/blktap/drivers/block-qcow.c	Thu Oct 30 19:50:59 2008 -0700
@@ -88,18 +88,8 @@
   int i;
   uint32_t md[4];
 
-  /* Convert L1 table to big endian */
-  for(i = 0; i < len / sizeof(uint64_t); i++) {
-    cpu_to_be64s(&((uint64_t*) ptr)[i]);
-  }
-
   /* Generate checksum */
   gcry_md_hash_buffer(GCRY_MD_MD5, md, ptr, len);
-
-  /* Convert L1 table back to native endianess */
-  for(i = 0; i < len / sizeof(uint64_t); i++) {
-    be64_to_cpus(&((uint64_t*) ptr)[i]);
-  }
 
   return md[0];
 }
@@ -117,21 +107,11 @@
   md = malloc(MD5_DIGEST_LENGTH);
   if(!md) return 0;
 
-  /* Convert L1 table to big endian */
-  for(i = 0; i < len / sizeof(uint64_t); i++) {
-    cpu_to_be64s(&((uint64_t*) ptr)[i]);
-  }
-
   /* Generate checksum */
   if (MD5((unsigned char *)ptr, len, md) != md)
     ret = 0;
   else
     memcpy(&ret, md, sizeof(uint32_t));
-
-  /* Convert L1 table back to native endianess */
-  for(i = 0; i < len / sizeof(uint64_t); i++) {
-    be64_to_cpus(&((uint64_t*) ptr)[i]);
-  }
 
   free(md);
   return ret;
@@ -491,7 +471,6 @@
 
 		/* update the L1 entry */
 		s->l1_table[l1_index] = l2_offset;
-		tmp = cpu_to_be64(l2_offset);
 		
 		/*Truncate file for L2 table 
 		 *(initialised to zero in case we crash)*/
@@ -874,22 +853,6 @@
 		(int) (s->l1_size * sizeof(uint64_t)), 
 		l1_table_size);
 
-	lseek(fd, 0, SEEK_SET);
-	l1_table_block = l1_table_size + s->l1_table_offset;
-	l1_table_block = l1_table_block + 512 - (l1_table_block % 512); 
-	ret = posix_memalign((void **)&buf2, 4096, l1_table_block);
-	if (ret != 0) goto fail;
-	if (read(fd, buf2, l1_table_block) != l1_table_block)
-		goto fail;
-	memcpy(s->l1_table, buf2 + s->l1_table_offset, l1_table_size);
-
-	for(i = 0; i < s->l1_size; i++) {
-		be64_to_cpus(&s->l1_table[i]);
-		//DPRINTF("L1[%d] => %llu\n", i, s->l1_table[i]);
-		if (s->l1_table[i] > final_cluster)
-			final_cluster = s->l1_table[i];
-	}
-
 	/* alloc L2 cache */
 	size = s->l2_size * L2_CACHE_SIZE * sizeof(uint64_t);
 	ret = posix_memalign((void **)&s->l2_cache, 4096, size);
@@ -911,32 +874,39 @@
 	
 	/*Detect min_cluster_alloc*/
 	s->min_cluster_alloc = 1; /*Default*/
-	if (s->backing_file_offset == 0 && s->l1_table_offset % 4096 == 0) {
+
+	l1_table_block = l1_table_size + s->l1_table_offset;
+	l1_table_block = (l1_table_block + 511) & ~511;
+	ret = posix_memalign((void **)&buf2, 512, l1_table_block);
+	if (ret != 0) goto fail;
+
+	lseek(fd, 0, SEEK_SET);
+	if (read(fd, buf2, l1_table_block) != l1_table_block)
+		goto fail;
+	memcpy(s->l1_table, buf2 + s->l1_table_offset, l1_table_size);
+
+	if (s->l1_table_offset % 4096 == 0) {
 		/*We test to see if the xen magic # exists*/
-		exthdr = (QCowHeader_ext *)(buf + sizeof(QCowHeader));
+		exthdr = (QCowHeader_ext *)(buf2 + sizeof(QCowHeader));
 		be32_to_cpus(&exthdr->xmagic);
+		be32_to_cpus(&exthdr->min_cluster_alloc);
+		be32_to_cpus(&exthdr->flags);
+
 		if(exthdr->xmagic != XEN_MAGIC) 
 			goto end_xenhdr;
-    
+		s->extended = 1;
+
 		/* 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 */
+		 * they don't use big endian but native endianess for the L1 table. *
+		 * After this block, the l1 table will definitely be in BIG endian.*/
 		if ((exthdr->flags & EXTHDR_L1_BIG_ENDIAN) == 0) {
 
 			/* 
-			   The image is broken. Fix it. The L1 table has already been 
-			   byte-swapped, so we can write it to the image file as it is
-			   currently in memory. Then swap it back to native endianess
-			   for operation.
+			   The image is broken. Fix it. Swap
+			   everything we've changed, and also the L1
+			   table to big endian for write to image.
 			 */
-
 			DPRINTF("qcow: Converting image to big endian L1 table\n");
-
-			memcpy(buf2 + s->l1_table_offset, s->l1_table, l1_table_size);
-			lseek(fd, 0, SEEK_SET);
-			if (write(fd, buf2, l1_table_block) != l1_table_block) {
-				DPRINTF("qcow: Failed to write new L1 table\n");
-				goto fail;
-			}
 
 			for(i = 0;i < s->l1_size; i++) {
 				cpu_to_be64s(&s->l1_table[i]);
@@ -944,11 +914,22 @@
 
 			/* Write the big endian flag to the extended header */
 			exthdr->flags |= EXTHDR_L1_BIG_ENDIAN;
-
-			if (write(fd, buf, 512) != 512) {
-				DPRINTF("qcow: Failed to write extended header\n");
+			
+			cpu_to_be32s(&exthdr->xmagic);
+			cpu_to_be32s(&exthdr->min_cluster_alloc);
+			cpu_to_be32s(&exthdr->flags);
+			
+			memcpy(buf2 + s->l1_table_offset, s->l1_table, l1_table_size);
+			lseek(fd, 0, SEEK_SET);
+			if (write(fd, buf2, l1_table_block) != l1_table_block) {
+				DPRINTF("qcow: Failed to write new L1 table\n");
 				goto fail;
 			}
+			
+			//Now we swap back to how it was before.
+			be32_to_cpus(&exthdr->xmagic);
+			be32_to_cpus(&exthdr->min_cluster_alloc);
+			be32_to_cpus(&exthdr->flags);
 		}
 
 		/*Finally check the L1 table cksum*/
@@ -958,14 +939,19 @@
 		if(exthdr->cksum != cksum)
 			goto end_xenhdr;
 			
-		be32_to_cpus(&exthdr->min_cluster_alloc);
-		be32_to_cpus(&exthdr->flags);
 		s->sparse = (exthdr->flags & SPARSE_FILE);
 		s->min_cluster_alloc = exthdr->min_cluster_alloc; 
 	}
 
  end_xenhdr:
-	
+	/* convert l1 table to native endian for operation */
+	for(i = 0; i < s->l1_size; i++) {
+		be64_to_cpus(&s->l1_table[i]);
+		//DPRINTF("L1[%d] => %llu\n", i, s->l1_table[i]);
+		if (s->l1_table[i] > final_cluster)
+			final_cluster = s->l1_table[i];
+	}
+
 	if (init_aio_state(driver)!=0) {
 	  DPRINTF("Unable to initialise AIO state\n");
 	  free_aio_state(s);
@@ -1127,24 +1113,62 @@
 
 	return;
 }
+
+static int
+tdqcow_update_checksum(struct tdqcow_state *s)
+{
+	int i, fd, err;
+	uint32_t offset, cksum, out;
+
+	if (!s->extended)
+		return 0;
+
+	fd = open(s->name, O_WRONLY | O_LARGEFILE); /* open without O_DIRECT */
+	if (fd == -1) {
+		err = errno;
+		goto out;
+	}
+
+	offset = sizeof(QCowHeader) + offsetof(QCowHeader_ext, cksum);
+	if (lseek(fd, offset, SEEK_SET) == (off_t)-1) {
+		err = errno;
+		goto out;
+	}
+
+	/* convert to big endian for checksum */
+	for (i = 0; i < s->l1_size; i++)
+		cpu_to_be64s(&s->l1_table[i]);
+
+	cksum = gen_cksum((char *)s->l1_table, s->l1_size * sizeof(uint64_t));
+
+	/* and back again... */
+	for (i = 0; i < s->l1_size; i++)
+		be64_to_cpus(&s->l1_table[i]);
+
+	DPRINTF("Writing cksum: %d", cksum);
+
+	out = cpu_to_be32(cksum);
+	if (write(fd, &out, sizeof(out)) != sizeof(out)) {
+		err = errno;
+		goto out;
+	}
+
+	err = 0;
+
+out:
+	if (err)
+		DPRINTF("failed to update checksum: %d\n", err);
+	if (fd != -1)
+		close(fd);
+	return err;
+}
  		
 int tdqcow_close(td_driver_t *driver)
 {
 	struct tdqcow_state *s = (struct tdqcow_state *)driver->data;
-	uint32_t cksum, out;
-	int fd, offset;
 
 	/*Update the hdr cksum*/
-	if(s->min_cluster_alloc == s->l2_size) {
-		cksum = gen_cksum((char *)s->l1_table, s->l1_size * sizeof(uint64_t));
-		printf("Writing cksum: %d",cksum);
-		fd = open(s->name, O_WRONLY | O_LARGEFILE); /*Open without O_DIRECT*/
-		offset = sizeof(QCowHeader) + sizeof(uint32_t);
-		lseek(fd, offset, SEEK_SET);
-		out = cpu_to_be32(cksum);
-		if (write(fd, &out, sizeof(uint32_t))) ;
-		close(fd);
-	}
+	tdqcow_update_checksum(s);
 
 	free(s->name);
 	free(s->l1_table);
@@ -1277,6 +1301,7 @@
 	} else
 		flags = SPARSE_FILE;
 
+	exthdr.flags |= EXTHDR_L1_BIG_ENDIAN;
 	exthdr.flags = cpu_to_be32(flags);
 	
 	/* write all the data */
diff -r 6eb1a5d782bf tools/blktap/drivers/img2qcow.c
--- a/tools/blktap/drivers/img2qcow.c	Thu Oct 30 19:48:08 2008 -0700
+++ b/tools/blktap/drivers/img2qcow.c	Thu Oct 30 19:50:59 2008 -0700
@@ -107,6 +107,7 @@
 
 static inline void LOCAL_FD_SET(fd_set *readfds) 
 {
+        FD_ZERO(readfds);
 	FD_SET(server.aio_queue.poll_fd, readfds);
 	maxfds = server.aio_queue.poll_fd;
 	
diff -r 6eb1a5d782bf tools/blktap/drivers/qcow.h
--- a/tools/blktap/drivers/qcow.h	Thu Oct 30 19:48:08 2008 -0700
+++ b/tools/blktap/drivers/qcow.h	Thu Oct 30 19:50:59 2008 -0700
@@ -60,6 +60,7 @@
 	char *name;                    /*Record of the filename*/
 	uint32_t backing_file_size;
 	uint64_t backing_file_offset;
+	uint8_t extended;              /*File contains extended header*/
 	int encrypted;                 /*File contents are encrypted or plain*/
 	int cluster_bits;              /*Determines length of cluster as 
 					*indicated by file hdr*/
@@ -89,8 +90,8 @@
 	uint32_t crypt_method_header;  /**/
 	AES_KEY aes_encrypt_key;       /*AES key*/
 	AES_KEY aes_decrypt_key;       /*AES key*/
+
         /* libaio state */
-
 	int                  aio_free_count;	
 	int                  max_aio_reqs;
 	struct qcow_request   *aio_requests;
diff -r 6eb1a5d782bf tools/blktap/drivers/qcow2raw.c
--- a/tools/blktap/drivers/qcow2raw.c	Thu Oct 30 19:48:08 2008 -0700
+++ b/tools/blktap/drivers/qcow2raw.c	Thu Oct 30 19:50:59 2008 -0700
@@ -112,6 +112,7 @@
 
 static inline void LOCAL_FD_SET(fd_set *readfds) 
 {
+        FD_ZERO(readfds);
 	FD_SET(qcowio_fd[0], readfds);
 	FD_SET(aio_fd[0], readfds);
 	
diff -r 6eb1a5d782bf tools/blktap/drivers/xmsnap
--- a/tools/blktap/drivers/xmsnap	Thu Oct 30 19:48:08 2008 -0700
+++ b/tools/blktap/drivers/xmsnap	Thu Oct 30 19:50:59 2008 -0700
@@ -54,11 +54,19 @@
 # Pause VM
 #
 xm pause $vmid
+if [ "$?" -ne "0" ]; then
+  exit 1
+fi
+
 
 #
 # Snap and reposition the files
 #
 mv $directory/$target $directory/$snap
+if [ "$?" -ne "0" ]; then
+  exit 1
+fi
+
 qcow-create 0 $directory/$target $directory/$snap
 
 #