[PATCH] nvme: reserve a keep-alive admin tag for all transports

[Keith Busch]

On Fri, May 08, 2026 at 11:04:27AM +0200, Christoph Hellwig wrote:
> On Tue, Apr 28, 2026 at 08:24:35AM +0100, Keith Busch wrote:
> > > This field specifies the timeout value for the Keep Alive feature in
> > > milliseconds.  [...]
> > > The default value for this field is 0h for NVMe transports that do not require use of the Keep Alive
> > > feature (e.g., NVMe over PCIe). For NVMe transports that require use of the Keep Alive feature
> > > (e.g., RDMA and TCP), the default value for this field is 1D4C0h "
> > > 
> > > To me, it sounds like for nvme-pci, keep alive isn't required, but could
> > > be activated.
> > 
> > The spec says the support is subject to the Transport binding
> > specification, which does not exist in the PCIe transport spec.
> 
> My memories from the fabrics working group back in the day is that we
> explicitly intended to support it in PCIe.  The wording in the spec
> referring to transport specs I can find is:
> 
> The NVMe Transport binding specification for the associated NVMe Transport
> defines:
> 
>  o the minimum Keep Alive Timeout value, if any;
>  o the maximum Keep Alive Timeout value, if any; and
>  o if the Keep Alive Timer feature is required to be supported and enabled.
> 
> which does not read to me like there is any required language in the
> transport spec to require keep alive.

So the absence of defining a minimum means it's simply optional? I
suppose I can see it that way as the intended interpretation, but seems
counter productive to do on PCIe when you can MMIO the controller status
register to verify liveness. If the controller responds successfully to
the feature, then I have to agree we need the host to do its part.