[PATCH] nvme-fabrics: handle zero MAXCMD without closing the connection

[Laurence Oberman]

On Fri, 2024-11-29 at 15:17 +0100, Maurizio Lombardi wrote:
> The NVMe specification states that MAXCMD is mandatory
> for NVMe-over-Fabrics implementations. However, some NVMe/TCP
> and NVMe/FC arrays from major vendors have buggy firmware
> that reports MAXCMD as zero in the Identify Controller data
> structure.
> 
> Currently, the implementation closes the connection in such cases,
> completely preventing the host from connecting to the target.
> 
> Fix the issue by printing a clear error message about the firmware
> bug
> and allowing the connection to proceed. It assumes that the
> target supports a MAXCMD value of SQSIZE + 1. If any issues arise,
> the user can manually adjust SQSIZE to mitigate them.
> 
> Fixes: 4999568184e5 ("nvme-fabrics: check max outstanding commands")
> Signed-off-by: Maurizio Lombardi <mlombard at redhat.com>
> ---
>  drivers/nvme/host/core.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index 1a8d32a4a5c3..91ee2c3aa95e 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -3257,8 +3257,9 @@ static int nvme_check_ctrl_fabric_info(struct
> nvme_ctrl *ctrl, struct nvme_id_ct
>         }
>  
>         if (!ctrl->maxcmd) {
> -               dev_err(ctrl->device, "Maximum outstanding commands
> is 0\n");
> -               return -EINVAL;
> +               dev_err(ctrl->device,
> +                       "Firmware bug: maximum outstanding commands
> is 0\n");
> +               ctrl->maxcmd = ctrl->sqsize + 1;
>         }
>  
>         return 0;

A similar fix was tested by a customer seeing this issue and they can
now connect to their array again.
Looks good.

Reviewed-by: Laurence Oberman <loberman at redhat.com>