Re: [RFC PATCH v2 2/6] x86/cpu: Use a new feature flag for 5 level paging

[Ingo Molnar <mingo@kernel.org>]

* Ard Biesheuvel <ardb@kernel.org> wrote:

> On Wed, 14 May 2025 at 09:04, Ingo Molnar <mingo@kernel.org> wrote:
> >
> >
> > * Kirill A. Shutemov <kirill@shutemov.name> wrote:
> >
> > > On Tue, May 13, 2025 at 01:12:00PM +0200, Ard Biesheuvel wrote:
> > > > From: Ard Biesheuvel <ardb@kernel.org>
> > > >
> > > > Currently, the LA57 CPU feature flag is taken to mean two different
> > > > things at once:
> > > > - whether the CPU implements the LA57 extension, and is therefore
> > > >   capable of supporting 5 level paging;
> > > > - whether 5 level paging is currently in use.
> > > >
> > > > This means the LA57 capability of the hardware is hidden when a LA57
> > > > capable CPU is forced to run with 4 levels of paging. It also means the
> > > > the ordinary CPU capability detection code will happily set the LA57
> > > > capability and it needs to be cleared explicitly afterwards to avoid
> > > > inconsistencies.
> > > >
> > > > Separate the two so that the CPU hardware capability can be identified
> > > > unambigously in all cases.
> > >
> > > Unfortunately, there's already userspace that use la57 flag in
> > > /proc/cpuinfo as indication that 5-level paging is active. :/
> > >
> > > See va_high_addr_switch.sh in kernel selftests for instance.
> >
> > Kernel selftests do not really count if that's the only userspace that
> > does this - but they indeed increase the likelihood that some external
> > userspace uses /proc/cpuinfo in that fashion. Does such external
> > user-space code exist?
> >
> 
> Bah, that seems likely if this is the only way user space is able to 
> infer that the kernel is using 5-level paging.

The price of past mistakes. :-/

So, the pragmatic, forward compatible solution would be to:

 - Keep the 'la57' user-visible flag in /proc/cpuinfo, but map it to 
   the X86_FEATURE_5LEVEL_PAGING flag internally.

 - Rename X86_FEATURE_LA57 to X86_FEATURE_LA57_HW, and expose it 
   as la57_hw.

This way, any LA57-supporting CPUs would always have la57_cpu set, 
while 'la57' is only set when it's enabled in the kernel.

An additional minor bonus would be that by renaming it to 
X86_FEATURE_LA57_HW, the change in behavior also becomes a bit more 
obvious at first glance to kernel developers.

Thanks,

	Ingo