linux-kernel - Re: [PATCH v2] nvme: allow 64-bit results in passthru commands

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Date:   Mon, 19 Aug 2019 15:21:45 -0600
From:   Keith Busch <kbusch@...nel.org>
To:     Sagi Grimberg <sagi@...mberg.me>
Cc:     Marta Rybczynska <mrybczyn@...ray.eu>,
        Christoph Hellwig <hch@....de>, axboe <axboe@...com>,
        linux-nvme <linux-nvme@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Samuel Jones <sjones@...ray.eu>,
        Guillaume Missonnier <gmissonnier@...ray.eu>
Subject: Re: [PATCH v2] nvme: allow 64-bit results in passthru commands

On Mon, Aug 19, 2019 at 02:17:44PM -0700, Sagi Grimberg wrote:
> 
> >>>> ----- On 16 Aug, 2019, at 15:16, Christoph Hellwig hch@....de wrote:
> >>>>> Sorry for not replying to the earlier version, and thanks for doing
> >>>>> this work.
> >>>>>
> >>>>> I wonder if instead of using our own structure we'd just use
> >>>>> a full nvme SQE for the input and CQE for that output.  Even if we
> >>>>> reserve a few fields that means we are ready for any newly used
> >>>>> field (at least until the SQE/CQE sizes are expanded..).
> >>>>
> >>>> We could do that, nvme_command and nvme_completion are already UAPI.
> >>>> On the other hand that would mean not filling out certain fields like
> >>>> command_id. Can do an approach like this.
> >>>
> >>> Well, we need to pass user space addresses and lengths, which isn't
> >>> captured in struct nvme_command.
> >>
> >> Isn't simply having a 64 variant simpler?
> > 
> > Could you provide more details on what you mean by this?
> 
> Why would we need to pass addresses and lengths if userspace is
> sending the 64 variant when it is expecting a 64 result?
> 
> Or maybe I'm missing something...

The recommendation was to have user space provide an SQE, i.e. 'struct
nvme_command', as input to the driver and receive 'struct nvme_completion'
in response. I am only pointing out that 'struct nvme_command' is
inappropriate for user space.