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

["Daniel P. Berrange" <berrange@redhat.com>]

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 <pl@kamp.de>
> > > > >>---
> > > > >>  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_BACKING_FORMAT 0xE2792ACA
> > > > >>  #define  QCOW2_EXT_MAGIC_FEATURE_TABLE 0x6803f857
> > > > >>+#define  QCOW2_EXT_MAGIC_COMPRESS_FORMAT 0xC03183A3
> > > > >>  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]) {
> > > > >>+        return QCOW2_COMPRESS_ZLIB_COMPAT;
> > > > >>+    } 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
> > > > >it.
> > > > 
> > > > 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.

Regards,
Daniel
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