Re: [PATCH] arm64: Add MIDR-based check for FEAT_ECBHB

[Oliver Upton <oliver.upton@linux.dev>]

On Mon, Jun 02, 2025 at 01:08:31PM +0100, Will Deacon wrote:
> On Thu, May 22, 2025 at 01:41:48PM -0700, Oliver Upton wrote:
> > +	/*
> > +	 * Prior to commit e8cde32f111f ("arm64/cpufeatures/kvm: Add ARMv8.9
> > +	 * FEAT_ECBHB bits in ID_AA64MMFR1 register"), KVM masked FEAT_ECBHB
> > +	 * on implementations that actually have the feature. That sucks; infer
> > +	 * presence of FEAT_ECBHB based on MIDR.
> > +	 */
> > +	if (is_midr_in_range_list(spectre_ecbhb_list))
> > +		return true;
> > +
> 
> I really don't think we want to go down this route.

Like I said, not a fan of doing this but...

> If finer grained control of the spectre mitigations is needed, I think
> extending the existing command-line options is probably the best bet
> rather then inferring behaviours based on the MIDR.

Looks like all of the Neoverse-V2 based VMs available for rent are
unintentionally hiding FEAT_ECBHB despite hardware support. I wouldn't
expect CSPs to go and change this field after creating a VM, so that's a
lot of hardware we're giving a poor experience on.

I just don't think a command-line switch is going to have any practical
impact on the situation.

Thanks,
Oliver