Re: [PATCH rdma-core 2/2] libhns: Support cq record doorbell

[Jason Gunthorpe <jgg-uk2M96/98Pc@public.gmane.org>]

On Thu, Jan 18, 2018 at 03:25:04PM +0800, Liuyixian (Eason) wrote:
> 
> 
> On 2018/1/18 12:37, Jason Gunthorpe wrote:
> > On Thu, Jan 18, 2018 at 10:36:01AM +0800, Yixian Liu wrote:
> > 
> >> diff --git a/providers/hns/hns_roce_u_verbs.c b/providers/hns/hns_roce_u_verbs.c
> >> index cde8568..7037a1c 100644
> >> +++ b/providers/hns/hns_roce_u_verbs.c
> >> @@ -276,6 +276,16 @@ struct ibv_cq *hns_roce_u_create_cq(struct ibv_context *context, int cqe,
> >>  
> >>  	cmd.buf_addr = (uintptr_t) cq->buf.buf;
> >>  
> >> +	if (to_hr_dev(context->device)->hw_version != HNS_ROCE_HW_VER1) {
> >> +		cq->set_ci_db = hns_roce_alloc_db(to_hr_ctx(context),
> >> +						  HNS_ROCE_CQ_TYPE_DB);
> >> +		if (!cq->set_ci_db) {
> >> +			fprintf(stderr, "alloc cq db buffer failed!\n");
> >> +			goto err_buf;
> >> +		}
> >> +		cmd.db_addr  = (uintptr_t) cq->set_ci_db;
> > 
> > Uhh.. why does the userspace already have the 'db_addr' member of
> > hns_roce_create_cq when the kernel doesn't?
> > 
> > What is going on here? How does forward and backwards compatibility of
> > the kABI work?
> > 
> > Jason
> >
> I have checked the history log, it seems that we have missed to add 'db_addr'
> for the kernel when adding it for the userspace.
> Up to now, we haven't referred this field in current driver both in kernel
> and userspace, that's why we haven't found this bug.
> 
> Thanks for your doubt!

Please explain how forward and backwards compatibility of
the kABI will work with this new db_addr capability.

Normally we'd rely on the kernel seeing that the udata is longer to
signal that userspace supports an optional feature, but that is broken
in this case.

Jason
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