Re: [Xen-devel] RING_HAS_UNCONSUMED_REQUESTS oddness

[Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>]

On 06/03/14 15:53, Ian Campbell wrote:
> On Thu, 2014-03-06 at 15:47 +0000, Zoltan Kiss wrote:
> > By my understanding, there is no way rsp could be smaller than req, so
> > there is no point having this. Am I missing something?
> It happens during wraparound, i.e. after req has wrapped but rsp hasn't
> yet.
The name of the macro suggest we are interested whether the ring has 
unconsumed requests, and netback uses it that way. The answer to that 
question is req_prod - req_cons. And it works if prod wrapped but cons 
didn't.
rsp calculates the number of "consumed but not responded" requests (it 
also works well if req_cons wrapped but rsp_prod_pvt didn't), then 
subtract it from the ring size. So it gives the number of unconsumed 
responses + unconsumed requests + unused slots. Why do we care about it? 
And as it includes the number of unconsumed requests, it couldn't be 
smaller than that, could it?
Zoli


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