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

[Klaus Jensen <its@irrelevant.dk>]

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On Nov  8 12:39, John Levon wrote:
> 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?
> 

Whelp.

Looks like I'm in the wrong here, apologies!

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