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 03:46:21PM +0200, Peter Lieven wrote:
> Am 10.07.2017 um 15:44 schrieb 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
> 
> Can you then please advise where to add the schematas and which functions
> to use for parsing it? You wrote that it is a bit hackish at the moment as not
> everything is available in qcow2_create path.

I just sent an (untested) example for patch 3 that should help. If you still
have problems let me know.

Regards,
Daniel
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