Re: [Qemu-devel] [Qemu-block] [PATCH v2 02/16] block-backend: Add blk_request_perm

[Fam Zheng <famz@redhat.com>]

On Thu, 05/11 15:32, Stefan Hajnoczi wrote:
> On Wed, Apr 19, 2017 at 05:43:42PM +0800, Fam Zheng wrote:
> > This function tries to request, if not granted yet, for the given
> > permissions.
> > 
> > Signed-off-by: Fam Zheng <famz@redhat.com>
> > ---
> >  block/block-backend.c          | 12 ++++++++++++
> >  include/sysemu/block-backend.h |  1 +
> >  2 files changed, 13 insertions(+)
> > 
> > diff --git a/block/block-backend.c b/block/block-backend.c
> > index 7405024..6bdd9ce 100644
> > --- a/block/block-backend.c
> > +++ b/block/block-backend.c
> > @@ -629,6 +629,18 @@ void blk_resume_after_migration(Error **errp)
> >      }
> >  }
> >  
> > +int blk_request_perm(BlockBackend *blk, uint64_t perm, Error **errp)
> > +{
> > +    uint64_t blk_perm, shared_perm;
> > +
> > +    blk_get_perm(blk, &blk_perm, &shared_perm);
> > +    if ((blk_perm & perm) == perm) {
> > +        return 0;
> > +    }
> > +    blk_perm |= perm;
> > +    return blk_set_perm(blk, blk_perm, shared_perm, errp);
> > +}
> 
> I'm slightly confused about why this function is needed.  blk_set_perm()
> doesn't do quite the right thing?

Sorry for the late reply!

I think blk_set_perm() always calls bs->drv->bdrv_check_perm() even when the
required perm bits are already acquired.

Fam