Re: [Qemu-devel] [PATCHv4 14/17] block/get_block_status: fix BDRV_BLOCK_ZERO for unallocated blocks

[Stefan Hajnoczi <stefanha@gmail.com>]

On Fri, Oct 18, 2013 at 02:49:11PM +0200, Paolo Bonzini wrote:
> Il 18/10/2013 14:38, Stefan Hajnoczi ha scritto:
> > On Tue, Oct 08, 2013 at 01:58:08PM +0200, Peter Lieven wrote:
> >> this patch does 2 things:
> >> a) only do additional call outs if BDRV_BLOCK_ZERO is not already set.
> >> b) use the newly introduced bdrv_has_discard_zeroes() to return the
> >>    zero state of an unallocated block. the used callout to
> >>    bdrv_has_zero_init() is only valid right after bdrv_create.
> >>
> >> Reviewed-by: Eric Blake <eblake@redhat.com>
> >> Signed-off-by: Peter Lieven <pl@kamp.de>
> >> ---
> >>  block.c |    4 ++--
> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/block.c b/block.c
> >> index fc931e3..1be4418 100644
> >> --- a/block.c
> >> +++ b/block.c
> >> @@ -3247,8 +3247,8 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
> >>          return ret;
> >>      }
> >>  
> >> -    if (!(ret & BDRV_BLOCK_DATA)) {
> >> -        if (bdrv_has_zero_init(bs)) {
> >> +    if (!(ret & BDRV_BLOCK_DATA) && !(ret & BDRV_BLOCK_ZERO)) {
> >> +        if (bdrv_has_discard_zeroes(bs)) {
> > 
> > I'm a little unclear about the semantics of bdrv_has_discard_zeroes().
> > Originally I thought it just meant any blocks discarded will read back
> > as zeroes.  But here it implies that any unallocated block reads
> > back as zeroes too?
> > 
> > In other words, this patch assumes unallocated blocks behave the same as
> > discarded blocks wrt to zeroes.
> 
> Note that earlier patches introduce both bdrv_has_discard_zeroes and
> bdrv_has_discard_write_zeroes.  There is no documentation, but the iscsi
> implementation let us understand the meaning:

There are doc comments but they differ from what you've posted:

+    /*
+     * Returns true if discarded blocks read back as zeroes.
+     */
+    bool (*bdrv_has_discard_zeroes)(BlockDriverState *bs);

> +static bool iscsi_has_discard_zeroes(BlockDriverState *bs)
> +{
> +    IscsiLun *iscsilun = bs->opaque;
> +    return !!iscsilun->lbprz;
> +}
> 
> That is, unallocated block reads back as zeroes

Okay, your semantics make sense.  With them the later patches are correct.

Peter: Please update the doc comments although and consider Paolo's comments
about block driver info.

Stefan