Re: [PATCH v3 4/4] hw/nvme: add polling support

[John Levon]

On Fri, Nov 04, 2022 at 07:32:12AM +0100, Klaus Jensen wrote:

> On Nov  3 21:19, Jinhao Fan wrote:
> > On 11/3/2022 8:10 PM, Klaus Jensen wrote:
> > > I agree that the spec is a little unclear on this point. In any case, in
> > > Linux, when the driver has decided that the sq tail must be updated,
> > > it will use this check:
> > > 
> > >    (new_idx - event_idx - 1) < (new_idx - old)
> > 
> > When eventidx is already behind, it's like:
> > 
> >  0
> >  1 <- event_idx
> >  2 <- old
> >  3 <- new_idx
> >  4
> >  .
> >  .
> >  .
> > 
> > In this case, (new_idx - event_idx - 1) = 3-1-1 = 1 >= (new_idx - old) =
> > 3-2=1, so the host won't update sq tail. Where am I wrong in this example?
> 
> That becomes 1 >= 1, i.e. "true". So this will result in the driver
> doing an mmio doorbell write.

The code is:

static inline int nvme_dbbuf_need_event(u16 event_idx, u16 new_idx, u16 old)    
     
{                                                                               
 
        return (u16)(new_idx - event_idx - 1) < (u16)(new_idx - old);           
 
}                                                                               
 

which per the above is "return 1 < 1;", or false. So the above case does *not*
do an mmio write. No?

regards
john