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

[Klaus Jensen]

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.

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