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

[Daniel P. Berrange]

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_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 

Regards,
Daniel
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