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Date:   Fri, 7 May 2021 13:40:52 -0700
From:   Keith Busch <kbusch@kernel.org>
To:     Sagi Grimberg <sagi@grimberg.me>
Cc:     Hannes Reinecke <hare@suse.de>,
        "Ewan D. Milne" <emilne@redhat.com>,
        Daniel Wagner <dwagner@suse.de>,
        linux-nvme@lists.infradead.org, linux-kernel@vger.kernel.org,
        Jens Axboe <axboe@fb.com>, Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v2] nvme-tcp: Check if request has started before
 processing it
Message-ID: <20210507204052.GA1485586@dhcp-10-100-145-180.wdc.com>
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On Fri, May 07, 2021 at 01:26:11PM -0700, Sagi Grimberg wrote:
> 
> > > > > I would also mention, that a crash is not even the scariest issue that
> > > > > we can see here, because if the request happened to be reused we are
> > > > > in the silent data corruption realm...
> > > > 
> > > > If this does happen, I think we have to come up with some way to
> > > > mitigate it. We're not utilizing the full 16 bits of the command_id, so
> > > > maybe we can append something like a generation sequence number that can
> > > > be checked for validity.
> > > 
> > > That's actually a great idea. scsi needs unique tags so it encodes the
> > > hwq in the upper 16 bits giving the actual tag the lower 16 bits which
> > > is more than enough for a single queue. We can do the same with
> > > a gencnt that will increment in both submission and completion and we
> > > can validate against it.
> > > 
> > > This will be useful for all transports, so maintaining it in
> > > nvme_req(rq)->genctr and introducing a helper like:
> > > rq = nvme_find_tag(tagset, cqe->command_id)
> > > That will filter genctr, locate the request.
> > > 
> > > Also:
> > > nvme_validate_request_gen(rq, cqe->command_id) that would
> > > compare against it.
> > > 
> > > 
> > > And then a helper to set the command_id like:
> > > cmd->common.command_id = nvme_request_command_id(rq)
> > > that will both increment the genctr and build a command_id
> > > from it.
> > > 
> > > Thoughts?
> > > 
> > 
> > Well, that would require a modification to the CQE specification, no?
> > fmds was not amused when I proposed that :-(
> 
> Why would that require a modification to the CQE? it's just using say
> 4 msbits of the command_id to a running sequence...

I think Hannes was under the impression that the counter proposal wasn't
part of the "command_id". The host can encode whatever it wants in that
value, and the controller just has to return the same value.