Re: [PATCH V8 06/44] mm/pkeys: Add Kconfig options for PKS

[Ira Weiny]

On Fri, Jan 28, 2022 at 02:54:26PM -0800, Dave Hansen wrote:
> On 1/27/22 09:54, ira.weiny@xxxxxxxxx wrote:
> > From: Ira Weiny <ira.weiny@xxxxxxxxx>
> > 
> > Protection Key Supervisor, PKS, is a feature used by kernel code only.
> > As such if no kernel users are configured the PKS code is unnecessary
> > overhead.

Indeed this was a bit weak sorry.  See below.

> > 
> > Define a Kconfig structure which allows kernel code to detect PKS
> > support by an architecture and then subsequently enable that support
> > within the architecture.
> > 
> > ARCH_HAS_SUPERVISOR_PKEYS indicates to kernel consumers that an
> > architecture supports pkeys.  PKS users can then select
> > ARCH_ENABLE_SUPERVISOR_PKEYS to turn on the support within the
> > architecture.
> > 
> > If ARCH_ENABLE_SUPERVISOR_PKEYS is not selected architectures avoid the
> > PKS overhead.
> > 
> > ARCH_ENABLE_SUPERVISOR_PKEYS remains off until the first kernel use case
> > sets it.
> 
> This is heavy on the "what" and weak on the "why".
> 
> Why isn't this an x86-specific Kconfig?  Why do we need two Kconfigs?
> Good old user pkeys only has one:
> 
> 	config ARCH_HAS_PKEYS
> 	        bool
> 
> and it's in arch-generic code because there are ppc and x86
> implementations *and* the pkey support touches generic code.
> 
> This might become evident later in the series, but it's clear as mud as
> it stands.

Sorry, I'll expand on this.

This issue is that because PKS users are in kernel only and are not part of the
architecture specific code there needs to be 2 mechanisms within the Kconfig
structure.  One to communicate an architectures support PKS such that the user
who needs it can depend on that config as well as a second to allow that user
to communicate back to the architecture to enable PKS.

Ira