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List: qemu-block
Subject: Re: [PATCH v3 4/4] hw/nvme: add polling support
From: John Levon <levon () movementarian ! org>
Date: 2022-11-08 12:39:55
Message-ID: Y2pOG89mnH3vGG/E () movementarian ! org
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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
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