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 <ebl...@redhat.com>
> >> Signed-off-by: Peter Lieven <p...@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
