Re: [PATCH v2 2/3] efi: arm64: Wire up BTI annotation in memory attributes table

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Feb 09, 2023 at 03:21:55PM +0100, Ard Biesheuvel wrote:
> On Wed, 8 Feb 2023 at 15:36, Ard Biesheuvel <ardb@kernel.org> wrote:
> > On Wed, 8 Feb 2023 at 15:25, Mark Rutland <mark.rutland@arm.com> wrote:
> > > I believe that there's no issue with mismatched CPUs, but there *might* might
> > > be a different issue with the ordering of feature detection and usage of the
> > > cap:
> > >
> > > * If CONFIG_ARM64_BTI_KERNEL=y, then the ARM64_BTI cap is detected as a strict
> > >   boot cpu feature, and secondaries without it will be rejected.
> > >
> > > * If CONFIG_ARM64_BTI_KERNEL=n then the ARM64_BTI cap is detected as a system
> > >   feature, and so we only set the cap bit after bringing all secondary CPUs
> > >   online, and only when *all* CPUs support it.
> > >
> > >   The happens under setup_cpu_features(), called from smp_cpus_done().
> > >
> > > So there's no issue with mismatch, but if system_supports_bti is called before
> > > smp_cpus_done() on a CONFIG_ARM64_BTI_KERNEL kernel it will return false. When
> > > do we set up the EFI mappings relative to that?
> > >
> >
> > Currently it is an early initcall so before SMP, but that is not
> > really necessary - the EFI table that carries this annotation is an
> > overlay that could easily be applied later.
> >
> > OTOH, what is the penalty for setting the GP attribute and using the
> > translation table on a core that does not implement BTI?
> 
> I'll merge this with the CONFIG_ARM64_BTI_KERNEL check re-added, if
> nobody minds?

That make sense to me; with the CONFIG_ARM64_BTI_KERNEL check:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.