Re: [PATCH] nvme: Use non zero KATO for persistent discovery connections

[Alistair Francis <alistair23@gmail.com>]

On Tue, Sep 2, 2025 at 8:35 PM Martin George <martinus.gpy@gmail.com> wrote:
>
> On Tue, 2025-09-02 at 13:52 +1000, alistair23@gmail.com wrote:
> > From: Alistair Francis <alistair.francis@wdc.com>
> >
> > The NVMe Base Specification 2.1 states that:
> >
> > """
> > A host requests an explicit persistent connection ... by specifying a
> > non-zero Keep Alive Timer value in the Connect command.
> > """
> >
> > As such if we are starting a persistent connection to a discovery
> > controller and the KATO is currently 0 we need to update KATO to a
> > non
> > zero value to avoid continuous timeouts on the target.
> >
> >
>
> When would this ever happen? Won't nvme-cli & nvme/host/fabrics.c in

It occurs if you perform a `nvme connect` to the discovery nqn of a
Linux target.

> the kernel ensure a PDC (persistent discovery controller) would always
> have the KATO either default set to NVMF_DEF_DISC_TMO (i.e. 30s) or any
> positive int value & not zero?

The kernel doesn't set a default for discovery controllers (hence this patch).

nvme-cli will only set NVMF_DEF_DISC_TMO if the `--persistent`
connection option is supplied to `nvme discover`. But it doesn't set a
KATO for `nvme connect`, even though it's a persistent connection.

Note, that I think that is a bug in nvme-cli and it should be setting
a non zero KATO. I plan on patching that. At the same time if the
kernel knows it's a persistent discovery connection it should also be
setting a default KATO. The kernel is already using a default non-zero
KATO for non discovery nqns (see nvmf_parse_options()). This just
extends the default to apply to persistent discovery controllers.

>
> Do you have a test log for the above scenario where the KATO ends up
> being zero for a PDC?

I do, it's just a lot of keep-alive timeout prints on the target

Alistair

>
> -Martin