linux-kernel - Re: [PATCH v2] nvme-tcp: Check if request has started before processing it

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Date:   Fri, 7 May 2021 17:03:08 -0700
From:   Keith Busch <kbusch@...nel.org>
To:     Sagi Grimberg <sagi@...mberg.me>
Cc:     Hannes Reinecke <hare@...e.de>,
        "Ewan D. Milne" <emilne@...hat.com>,
        Daniel Wagner <dwagner@...e.de>,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...com>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2] nvme-tcp: Check if request has started before
 processing it

On Fri, May 07, 2021 at 04:22:30PM -0700, Sagi Grimberg wrote:
> 
> > > > 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.
> 
> Yea, maybe something like this?

Yes, this looks aligned with what I was thinking. Just one minor problem
below.

> diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
> index 05f31a2c64bb..96abfb0e2ddd 100644
> --- a/drivers/nvme/host/nvme.h
> +++ b/drivers/nvme/host/nvme.h
> @@ -158,6 +158,7 @@ enum nvme_quirks {
>  struct nvme_request {
>         struct nvme_command     *cmd;
>         union nvme_result       result;
> +       u8                      genctr;
>         u8                      retries;
>         u8                      flags;
>         u16                     status;
> @@ -497,6 +498,48 @@ struct nvme_ctrl_ops {
>         int (*get_address)(struct nvme_ctrl *ctrl, char *buf, int size);
>  };
> 
> +/*
> + * nvme command_id is constructed as such:
> + * | xxxx | xxxxxxxxxxxx |
> + *   gen    request tag
> + */
> +#define nvme_cid_install_genctr(gen)           ((gen & 0xf) << 12)
> +#define nvme_genctr_from_cid(cid)              ((cid & 0xf000) >> 12)
> +#define nvme_tag_from_cid(cid)                 (cid & 0xfff)
> +
> +static inline u16 nvme_cid(struct request *rq)
> +{
> +       return nvme_cid_install_genctr(nvme_req(rq)->genctr++) | rq->tag;
> +}
> +
> +static inline struct request *nvme_find_rq(struct blk_mq_tags *tags,
> +               u16 command_id)
> +{
> +       u8 genctr = nvme_genctr_from_cid(command_id);
> +       u16 tag = nvme_tag_from_cid(command_id);
> +       struct request *rq;
> +
> +       rq = blk_mq_tag_to_rq(tags, tag);
> +       if (unlikely(!rq)) {
> +               pr_err("could not locate request for tag %#x\n",
> +                       tag);
> +               return NULL;
> +       }
> +       if (unlikely(nvme_req(rq)->genctr != genctr)) {

The nvme_request 'genctr' is 8 bits, but the nvme_genctr_from_cid() can
only return 4 bits, so need to use the same mask for both.