Re: [PATCH v4 3/5] arm64: Add support for FEAT_HAFT

[Marc Zyngier <maz@kernel.org>]

On Mon, 04 Nov 2024 17:28:48 +0000,
Catalin Marinas <catalin.marinas@arm.com> wrote:
> 
> On Sat, Nov 02, 2024 at 06:42:33PM +0800, Yicong Yang wrote:
> > diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> > index 3d261cc123c1..ed8c784ca082 100644
> > --- a/arch/arm64/include/asm/cpufeature.h
> > +++ b/arch/arm64/include/asm/cpufeature.h
> > @@ -838,6 +838,12 @@ static inline bool system_supports_poe(void)
> >  		alternative_has_cap_unlikely(ARM64_HAS_S1POE);
> >  }
> >  
> > +static inline bool system_supports_haft(void)
> > +{
> > +	return IS_ENABLED(CONFIG_ARM64_HAFT) &&
> > +		cpus_have_final_cap(ARM64_HAFT);
> > +}
> 
> I'm fine with this approach. If we ever get hardware with mismatched
> FEAT_HAFT and some secondary CPUs don't come up, we can revisit.
> 
> > diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> > index ccbae4525891..0bc88df7cb35 100644
> > --- a/arch/arm64/mm/proc.S
> > +++ b/arch/arm64/mm/proc.S
> > @@ -498,6 +498,10 @@ alternative_else_nop_endif
> >  	and	x9, x9, ID_AA64MMFR1_EL1_HAFDBS_MASK
> >  	cbz	x9, 1f
> >  	orr	tcr, tcr, #TCR_HA		// hardware Access flag update
> > +
> > +#ifdef CONFIG_ARM64_HAFT
> > +	orr	tcr2, tcr2, TCR2_EL1x_HAFT
> > +#endif /* CONFIG_ARM64_HAFT */
> >  1:
> >  #endif	/* CONFIG_ARM64_HW_AFDBM */
> >  	msr	mair_el1, mair
> 
> If you still want #ifdefs, I'd have left it outside the HW_AFDBM. We
> already have a dependency in the Kconfig. Anyway, I can fix this up.
> 
> I think as an additional patch we can also remove the ID checks for the
> tcr bit in tge HW_AFDBM case. But that's unrelated to this series.

I think you want to be careful with this one. I know of at least one
implementation that has a broken FEAT_HAFDBS implementation, that
removes it from the ID registers, but where the control bit in TCR_ELx
still takes effect.

Please see 6df696cd9bc1 ("arm64: errata: Mitigate Ampere1 erratum
AC03_CPU_38 at stage-2") which indicates how we actually rely on the
check for S1 translation.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.