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SubjectRe: [PATCH v4 01/13] Linux RDMA Core Changes
> > > 
> > > It seems all Chelsio needs is to pass in a consumer index - so, how about a new
> > > entry point? Something like void set_cq_udata(struct ib_cq *cq, struct ib_udata *udata)?
> > >
> >
> > Adding a new entry point would hurt chelsio's user mode performance if
> > if then requires 2 kernel transitions to rearm the cq.
> No, it won't need 2 transitions - just an extra function call,
> so it won't hurt performance - it would improve performance.
> ib_uverbs_req_notify_cq would call
> ib_uverbs_req_notify_cq()
> {
> ib_set_cq_udata(cq, udata)
> ib_req_notify_cq(cq, cmd.solicited_only ?
> }

ib_set_cq_udata() would transition into the kernel to pass in the
consumer's index. In addition, ib_req_notify_cq would also transition
into the kernel since its not a bypass function for chelsio.

> This way kernel consumers don't incur any overhead,
> and in userspace users extra function call is dwarfed
> by system call overhead.
> > Passing in user data is sort of SOP for these sorts of verbs.
> I don't see other examples. Where we did pass extra user data
> is in non-data pass verbs such as create QP.
> This is most inner tight loop in many ULPs, so we should be very careful
> about adding code there - these things do add up.
> See recent IRQ API update in kernel.

Roland, do you have any comments on this? You previously indicated
these patches were good to go once chelsio's ethernet driver gets pulled

> > How much does passing one more param cost for kernel users?
> Donnu. I just reviewed the code.
> It really should be up to patch submitter to check the performance
> effect of his patch, if there might be any.

I've run this code with mthca and didn't notice any performance
degradation, but I wasn't specifically measuring cq_poll overhead in a
tight loop...

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