Re: [Qemu-devel] [PATCH v3 2/6] block/nvme: fix doorbell stride

[Maxim Levitsky]

On Fri, 2019-07-05 at 13:10 +0200, Max Reitz wrote:
> On 05.07.19 13:09, Max Reitz wrote:
> > On 03.07.19 17:59, Maxim Levitsky wrote:
> > > Fix the math involving non standard doorbell stride
> > > 
> > > Signed-off-by: Maxim Levitsky <address@hidden>
> > > ---
> > >  block/nvme.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/block/nvme.c b/block/nvme.c
> > > index 6d4e7f3d83..52798081b2 100644
> > > --- a/block/nvme.c
> > > +++ b/block/nvme.c
> > > @@ -217,7 +217,7 @@ static NVMeQueuePair 
> > > *nvme_create_queue_pair(BlockDriverState *bs,
> > >          error_propagate(errp, local_err);
> > >          goto fail;
> > >      }
> > > -    q->cq.doorbell = &s->regs->doorbells[idx * 2 * s->doorbell_scale + 
> > > 1];
> > > +    q->cq.doorbell = &s->regs->doorbells[(idx * 2 + 1) * 
> > > s->doorbell_scale];
> > >  
> > >      return q;
> > >  fail:
> > 
> > Hm.  How has this ever worked?
> 
> (Ah, because CAP.DSTRD has probably been 0 in most devices.)
> 
Exactly, and I used cache line stride in my nvme-mdev, which broke this and I 
spend an evening figuring out
what is going on. I was sure that there is some memory ordering bug or 
something even weirder before (as usual)
finding that this is a very simple bug.
I tested nvme-mdev pretty much with everything I could get my hands on, 
including this driver.


Best regards,
        Maxim Levitsky