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

[Ingo Molnar <mingo@kernel.org>]

* Ingo Molnar <mingo@kernel.org> wrote:

> 
> * 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.

s/would always have la57_hw set

> 
> 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