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

[Will Deacon]

On Wed, Oct 28, 2020 at 11:49:46AM +0000, Catalin Marinas wrote:
> 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).

Hmm, but that's exactly what this cmdline option is about. We pretend that
the system has 32-bit EL0 when normally we would say that we don't.

> 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.

I really don't think this gains us anything. The current users of
system_supports_32bit_el0() are:

  - The ELF loader
  - CPU feature sanitisation code
  - Personality syscall
  - KVM

and, afaict, all of these would need to check the new feature if we added
it.  I think it would also mean that at least one 32-bit capable CPU would
have to boot early in order for the new feature to be advertised, which
feels like an artificial restriction to me, particularly as you could just
offline it immediately.

That said, I have dropped the print (see below) because the whole
"Detected" part is clearly bogus.

Will

--->8

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 813362a91995..ac6aff3a69de 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -1264,7 +1264,12 @@ device_initcall(aarch32_el0_sysfs_init);
 
 static bool has_32bit_el0(const struct arm64_cpu_capabilities *entry, int scope)
 {
-       return has_cpuid_feature(entry, scope) || __allow_mismatched_32bit_el0;
+       if (has_cpuid_feature(entry, scope)) {
+               pr_info("detected: 32-bit EL0 Support\n");
+               return true;
+       }
+
+       return __allow_mismatched_32bit_el0;
 }
 
 static bool has_useable_gicv3_cpuif(const struct arm64_cpu_capabilities *entry, int scope)
@@ -1874,7 +1879,6 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
        },
 #endif /* CONFIG_ARM64_VHE */
        {
-               .desc = "32-bit EL0 Support",
                .capability = ARM64_HAS_32BIT_EL0,
                .type = ARM64_CPUCAP_SYSTEM_FEATURE,
                .matches = has_32bit_el0,