Re: [PATCH v9 1/3] block: introduce compress filter driver

[Max Reitz]

On 26.11.19 15:43, Andrey Shinkevich wrote:
> Allow writing all the data compressed through the filter driver.
> The written data will be aligned by the cluster size.
> Based on the QEMU current implementation, that data can be written to
> unallocated clusters only. May be used for a backup job.
> 
> Suggested-by: Max Reitz <address@hidden>
> Signed-off-by: Andrey Shinkevich <address@hidden>
> ---
>  block/Makefile.objs     |   1 +
>  block/filter-compress.c | 190 
> ++++++++++++++++++++++++++++++++++++++++++++++++
>  qapi/block-core.json    |  10 ++-
>  3 files changed, 197 insertions(+), 4 deletions(-)
>  create mode 100644 block/filter-compress.c

[...]

> diff --git a/block/filter-compress.c b/block/filter-compress.c
> new file mode 100644
> index 0000000..ef4b12b
> --- /dev/null
> +++ b/block/filter-compress.c

[...]

> +#define PERM_PASSTHROUGH (BLK_PERM_CONSISTENT_READ \
> +                          | BLK_PERM_WRITE \
> +                          | BLK_PERM_RESIZE)
> +#define PERM_UNCHANGED (BLK_PERM_ALL & ~PERM_PASSTHROUGH)
> +
> +static void compress_child_perm(BlockDriverState *bs, BdrvChild *c,
> +                                const BdrvChildRole *role,
> +                                BlockReopenQueue *reopen_queue,
> +                                uint64_t perm, uint64_t shared,
> +                                uint64_t *nperm, uint64_t *nshared)
> +{
> +    *nperm = perm & PERM_PASSTHROUGH;
> +    *nshared = (shared & PERM_PASSTHROUGH) | PERM_UNCHANGED;
> +
> +    /*
> +     * We must not request write permissions for an inactive node, the child
> +     * cannot provide it.
> +     */
> +    if (!(bs->open_flags & BDRV_O_INACTIVE)) {
> +        *nperm |= BLK_PERM_WRITE_UNCHANGED;
> +    }

The copy-on-read filter has to take the WRITE_UNCHANGED permission
because it will do such writes for every read, but I don’t think this
driver ever needs to take this permission.  Therefore it should be
enough to use bdrv_filter_default_perms for .bdrv_child_perm.

Max

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