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

[Marc Zyngier <maz@kernel.org>]

On Tue, 05 Nov 2024 09:58:26 +0000,
Catalin Marinas <catalin.marinas@arm.com> wrote:
> 
> On Tue, Nov 05, 2024 at 08:35:51AM +0000, Marc Zyngier wrote:
> > 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/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.
> 
> Ah, thanks for this. So the hardware with the erratum above can still
> update the pte after it has been marked invalid, hence we can't turn it
> on in TCR_EL1 even if the rest of the kernel considers the feature
> disabled. So yes, the HAFDBS code needs to stay as is.

Indeed. Atomicity is overrated, let's go shopping.

> Let's hope the hardware people learnt and we won't have similar errata
> for FEAT_HAFT.

If I was religious, I'd light a candle. But we've both seen enough HW
to know that they *will* fsck it up. We just don't know how yet.

	M.

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