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

[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.

Kevin