netdev - Re: [PATCH rdma-next 00/10] Enable relaxed ordering for ULPs

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Date:   Fri, 9 Apr 2021 13:40:46 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Tom Talpey <tom@...pey.com>
Cc:     Chuck Lever III <chuck.lever@...cle.com>,
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Subject: Re: [PATCH rdma-next 00/10] Enable relaxed ordering for ULPs

On Fri, Apr 09, 2021 at 10:26:21AM -0400, Tom Talpey wrote:

> My belief is that the biggest risk is from situations where completions
> are batched, and therefore polling is used to detect them without
> interrupts (which explicitly).

We don't do this in the kernel.

All kernel ULPs only read data after they observe the CQE. We do not
have "last data polling" and our interrupt model does not support some
hacky "interrupt means go and use the data" approach.

ULPs have to be designed this way to use the DMA API properly.
Fencing a DMA before it is completed by the HW will cause IOMMU
errors.

Userspace is a different story, but that will remain as-is with
optional relaxed ordering.

Jason