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

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Date:   Mon, 19 Aug 2019 08:57:01 -0700
From:   James Smart <james.smart@...adcom.com>
To:     Keith Busch <kbusch@...nel.org>,
        Marta Rybczynska <mrybczyn@...ray.eu>
Cc:     Sagi Grimberg <sagi@...mberg.me>, Samuel Jones <sjones@...ray.eu>,
        Guillaume Missonnier <gmissonnier@...ray.eu>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-nvme <linux-nvme@...ts.infradead.org>,
        axboe <axboe@...com>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2] nvme: allow 64-bit results in passthru commands



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@....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