[PATCH 2/6] arm64: Allow mismatched 32-bit EL0 support

[Catalin Marinas]

On Wed, Oct 28, 2020 at 11:23:43AM +0000, Will Deacon wrote:
> On Wed, Oct 28, 2020 at 11:22:06AM +0000, Catalin Marinas wrote:
> > On Wed, Oct 28, 2020 at 11:17:13AM +0000, Will Deacon wrote:
> > > On Wed, Oct 28, 2020 at 11:12:04AM +0000, Catalin Marinas wrote:
> > > > On Tue, Oct 27, 2020 at 09:51:14PM +0000, Will Deacon wrote:
> > > > > +static bool has_32bit_el0(const struct arm64_cpu_capabilities *entry, int scope)
> > > > > +{
> > > > > +	return has_cpuid_feature(entry, scope) || __allow_mismatched_32bit_el0;
> > > > > +}
> > > > > +
> > > > >  static bool has_useable_gicv3_cpuif(const struct arm64_cpu_capabilities *entry, int scope)
> > > > >  {
> > > > >  	bool has_sre;
> > > > > @@ -1803,7 +1851,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
> > > > >  		.desc = "32-bit EL0 Support",
> > > > >  		.capability = ARM64_HAS_32BIT_EL0,
> > > > >  		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
> > > > > -		.matches = has_cpuid_feature,
> > > > > +		.matches = has_32bit_el0,
> > > > 
> > > > Ah, so this one reports 32-bit EL0 support even if no CPU actually
> > > > supports 32-bit (passing the command line option on TX2 would come up
> > > > with 32-bit EL0 in dmesg). I'd rather hide the .desc above and print the
> > > > information elsewhere when have at least one CPU supporting this.
> > > 
> > > Yeah, the problem is if a CPU with 32-bit EL0 support was late-onlined,
> > > then we would have 32-bit support, so I think this is an oddity that you
> > > get when the command line is passed. That said, I could nobble .desc and
> > > print it from the .matches function, with a slightly different message
> > > when the command line is passed.
> > 
> > I think we could do a pr_info_once() in update_32bit_cpu_features().
> 
> Is that called on a system with one CPU?

Ah, it's not.

Anyway, I see your reasoning behind the late CPUs but I don't
particularly like abusing the cpufeature support to pretend a
SYSTEM_FEATURE is available before knowing any CPU has it (maybe we do
it in other cases, I haven't checked).

Could we not instead add a new feature for asymmetric support that's
defined as ARM64_CPUCAP_WEAK_LOCAL_CPU_FEATURE? This would be allowed
for late CPUs and we keep the system_supports_32bit_el0() unchanged.

-- 
Catalin