Re: [Qemu-devel] [PATCH v2 08/17] blkdebug: Set request_alignment during .bdrv_refresh_limits()

[Kevin Wolf]

Am 14.06.2016 um 23:30 hat Eric Blake geschrieben:
> We want to eventually stick request_alignment alongside other
> BlockLimits, but first, we must ensure it is populated at the
> same time as all other limits, rather than being a special case
> that is set only when a block is first opened.
> 
> qemu-iotests 77 is particularly sensitive to the fact that we
> can specify an artificial alignment override in blkdebug, and
> that override must continue to work even when limits are
> refreshed on an already open device.
> 
> A later patch will be altering when bs->request_alignment
> defaults are set, so fall back to sector alignment if we have
> not inherited anything yet.
> 
> Signed-off-by: Eric Blake <address@hidden>
> 
> ---
> v2: new patch
> ---
>  block/blkdebug.c | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/block/blkdebug.c b/block/blkdebug.c
> index 20d25bd..1589fa7 100644
> --- a/block/blkdebug.c
> +++ b/block/blkdebug.c
> @@ -37,6 +37,7 @@
>  typedef struct BDRVBlkdebugState {
>      int state;
>      int new_state;
> +    int align;
> 
>      QLIST_HEAD(, BlkdebugRule) rules[BLKDBG__MAX];
>      QSIMPLEQ_HEAD(, BlkdebugRule) active_rules;
> @@ -382,9 +383,10 @@ static int blkdebug_open(BlockDriverState *bs, QDict 
> *options, int flags,
>      }
> 
>      /* Set request alignment */
> -    align = qemu_opt_get_size(opts, "align", bs->request_alignment);
> -    if (align > 0 && align < INT_MAX && !(align & (align - 1))) {
> -        bs->request_alignment = align;
> +    align = qemu_opt_get_size(opts, "align",
> +                              bs->request_alignment ?: BDRV_SECTOR_SIZE);

In the state as of this patch: How would bs->request_alignment ever be
zero? It is always initialised as 512 (because blkdebug doesn't
implement byte-based interfaces).

Later in this series, you move the initialisation of the field to
bdrv_refresh_limits(). However, the check still doesn't make sense
because now it's always 0 and you always use the BDRV_SECTOR_SIZE
fallback.

I think you should either just unconditionally use BDRV_SECTOR_SIZE or
even better just store 0 in s->align if the option isn't given. Your
implementation of blkdebug_refresh_limits() already leaves the default
bs->request_alignment alone if s->align == 0.

> +    if (align > 0 && align < INT_MAX && is_power_of_2(align)) {
> +        s->align = align;
>      } else {
>          error_setg(errp, "Invalid alignment");
>          ret = -EINVAL;

Kevin