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Subject: Re: [PATCH v2] nvme: allow 64-bit results in passthru commands
To:     Keith Busch <kbusch@kernel.org>,
        Marta Rybczynska <mrybczyn@kalray.eu>
Cc:     Sagi Grimberg <sagi@grimberg.me>, Samuel Jones <sjones@kalray.eu>,
        Guillaume Missonnier <gmissonnier@kalray.eu>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-nvme <linux-nvme@lists.infradead.org>,
        axboe <axboe@fb.com>, Christoph Hellwig <hch@lst.de>
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From:   James Smart <james.smart@broadcom.com>
Message-ID: <54e7a40d-a06c-a777-7061-0503051cc6bf@broadcom.com>
Date:   Mon, 19 Aug 2019 08:57:01 -0700
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On 8/19/2019 7:49 AM, Keith Busch wrote:
> On Mon, Aug 19, 2019 at 12:06:23AM -0700, Marta Rybczynska wrote:
>> ----- On 16 Aug, 2019, at 15:16, Christoph Hellwig hch@lst.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.
>
This is going to be fun.  It's going to have to be a cooperative effort 
between app and transport. There will always need to be some parts of 
the SQE filled out by the transport like SGL, command type/subtype bits, 
as well as dealing with buffers as Keith states. Command ID is another 
of those fields.

-- james