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

[Peter Lieven]

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_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.

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

Thanks,
Peter