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

[Chao S]

On Fri, May 8, 2026, Keith Busch wrote:
> 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.

Thanks Keith - I will take that last sentence as the basis for v2.
The v2 commit message will cite the spec reading (NVMe 2.0a 5.27.1.12
plus the transport binding wording) explicitly and conclude that PCIe
MAY support KATO, in which case the host needs the reserved tag.

For data points: I also tested two Micron PCIe SSDs locally and both
report KAS=0, so the bug is likely field-rare on mainstream hardware
today.  The patch remains a cheap defensive change for spec-compliant
controllers that declare KAS != 0 or for emulated environments.

Chao

On Fri, May 8, 2026 at 5:31 AM Keith Busch <kbusch@xxxxxxxxxx> wrote:
>
> 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.