[Libguestfs] [RFC libnbd PATCH] info: Add support for new qemu:joint-allocation

[Nir Soffer]

On Thu, Jun 10, 2021 at 12:32 AM Eric Blake <eblake at redhat.com> wrote:
>
> Qemu is adding qemu:joint-allocation as a single context combining the
> two bits of base:allocation and a compression of qemu:allocation-depth
> into two bits [1].  Decoding the bits makes it easier for humans to
> see the result of that context.
>
> [1] https://lists.gnu.org/archive/html/qemu-devel/2021-06/msg02446.html
> ---
>
> Obviously, this libnbd patch should only go in if the qemu RFC is
> accepted favorably.  With this patch applied, the example listed in my
> qemu patch 2/2 commit message [2] becomes
>
> $ ~/libnbd/run nbdinfo --map=qemu:joint-allocation nbd://localhost
>          0       65536    3  hole,zero,unallocated
>      65536       65536    4  allocated,local
>     131072       65536    7  hole,zero,local
>     196608       65536    3  hole,zero,unallocated
>
> [2] https://lists.gnu.org/archive/html/qemu-devel/2021-06/msg02448.html
>
> For what it's worth, you can also play with the qemu+libnbd patches at:
> https://repo.or.cz/qemu/ericb.git/ master
> https://repo.or.cz/libnbd/ericb.git/ master
>
> (I sometimes rewind those branches, but they'll be stable for at least
> a few days after this email)
>
>  info/map.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>
> diff --git a/info/map.c b/info/map.c
> index ae6d4fe..21e8657 100644
> --- a/info/map.c
> +++ b/info/map.c
> @@ -226,6 +226,27 @@ extent_description (const char *metacontext, uint32_t type)
>        return ret;
>      }
>    }
> +  else if (strcmp (metacontext, "qemu:joint-allocation") == 0) {
> +    /* Combo of base:allocation and stripped-down qemu:allocation-depth */
> +    const char *base, *depth;
> +    switch (type & 3) {
> +    case 0: base = "allocated"; break;
> +    case 1: base = "hole"; break;
> +    case 2: base = "zero"; break;
> +    case 3: base = "hole,zero"; break;
> +    }
> +    switch (type & 0xc) {
> +    case 0: depth = "unallocated"; break;

Is this possible? qemu reports BDRV_BLOCK_DATA but not BDRV_BLOCK_ALLOCATED?

Anyway this seems like a valid way to present qemu response.

> +    case 4: depth = "local"; break;
> +    case 8: depth = "backing"; break;
> +    case 12: depth = "<unexpected depth>"; break;

This should not be possible based on the qemu patch, but printing this
seems like a good solution, and can help to debug such an issue.

Thinking about client code trying to copy extents based on the flags,
the client should abort the operation since qemu response is invalid.

> +    }
> +    if (asprintf (&ret, "%s,%s", base, depth) == -1) {
> +      perror ("asprintf");
> +      exit (EXIT_FAILURE);
> +    }
> +    return ret;
> +  }
>
>    return NULL;   /* Don't know - description field will be omitted. */
>  }
> --
> 2.31.1
>