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Re: [Qemu-block] [PATCH V2 5/8] block/qcow2: read and write the compress

From: Peter Lieven
Subject: Re: [Qemu-block] [PATCH V2 5/8] block/qcow2: read and write the compress format extension
Date: Thu, 13 Jul 2017 10:44:53 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1

Am 10.07.2017 um 15:55 schrieb Daniel P. Berrange:
On Mon, Jul 10, 2017 at 03:52:18PM +0200, Kevin Wolf wrote:
Am 10.07.2017 um 15:44 hat Daniel P. Berrange geschrieben:
On Mon, Jul 10, 2017 at 03:34:59PM +0200, Kevin Wolf wrote:
Am 10.07.2017 um 15:29 hat Peter Lieven geschrieben:
Am 10.07.2017 um 15:25 schrieb Kevin Wolf:
Am 29.06.2017 um 12:57 hat Peter Lieven geschrieben:
we now read the extension on open and write it on update, but
do not yet use it.

Signed-off-by: Peter Lieven <address@hidden>
  block/qcow2.c | 100 ++++++++++++++++++++++++++++++++++++++++++++++++++--------
  block/qcow2.h |  23 +++++++++++---
  2 files changed, 104 insertions(+), 19 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 308121a..39a8afc 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -63,6 +63,7 @@ typedef struct {
  #define  QCOW2_EXT_MAGIC_END 0
  #define  QCOW2_EXT_MAGIC_FEATURE_TABLE 0x6803f857
  static int qcow2_probe(const uint8_t *buf, int buf_size, const char *filename)
@@ -76,6 +77,26 @@ static int qcow2_probe(const uint8_t *buf, int buf_size, 
const char *filename)
          return 0;
+static int qcow2_compress_format_from_name(char *fmt)
+    if (!fmt || !fmt[0]) {
+    } else if (g_str_equal(fmt, "zlib")) {
+        return QCOW2_COMPRESS_ZLIB;
+    } else {
+        return -EINVAL;
+    }
It might make sense to allow specifying the old compression format in
the compression header. But I'm not so sure about using the empty string
rather than a proper name for it, and even less about not documenting
The old format is used if and only if the compression header is absent.
It makes no sense to write a header and use the old format. The old
settings are suboptimal and old versions can't open a qcow2 with a
compression header anyway.
Your code allows an empty string in the header extension. If you don't
want it there, you need to reject it.
This is a good reason to use the QAPI schema to parse the create options
instead of doing it using qemu_opts - the QAPI schema will generate correct
code to parse & validate enum values
I'd really like to see QAPI used for bdrv_create(), but here we're in
the bdrv_open() path and parsing qcow2 headers. So either way this one
specifically wouldn't be fixed.
Sorry, yes, I was getting mixed up in the two different patches with
similar bits of code.

We could still replace the qcow2_compress_format_from_name() method
though with a call to the qapi_enum_parse() passing in the
'QcowCompressFormat_lookup' table.

Can you advise how exactly I would do that? Sorry, but this is new stuff for me.


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