Re: [PATCH v3 01/16] arm64: cpufeature: add pointer auth meta-capabilities

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Jan 15, 2020 at 01:52:24PM +0000, Suzuki K Poulose wrote:
> On 15/01/2020 12:26, Catalin Marinas wrote:
> > On Mon, Dec 16, 2019 at 02:17:03PM +0530, Amit Daniel Kachhap wrote:
> > > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > > index 04cf64e..cf42c46 100644
> > > --- a/arch/arm64/kernel/cpufeature.c
> > > +++ b/arch/arm64/kernel/cpufeature.c
> > > @@ -1249,6 +1249,20 @@ static void cpu_enable_address_auth(struct arm64_cpu_capabilities const *cap)
> > >   	sysreg_clear_set(sctlr_el1, 0, SCTLR_ELx_ENIA | SCTLR_ELx_ENIB |
> > >   				       SCTLR_ELx_ENDA | SCTLR_ELx_ENDB);
> > >   }
> > > +
> > > +static bool has_address_auth(const struct arm64_cpu_capabilities *entry,
> > > +			     int __unused)
> > > +{
> > > +	return cpus_have_const_cap(ARM64_HAS_ADDRESS_AUTH_ARCH) ||
> > > +	       cpus_have_const_cap(ARM64_HAS_ADDRESS_AUTH_IMP_DEF);
> > > +}
> > > +
> > > +static bool has_generic_auth(const struct arm64_cpu_capabilities *entry,
> > > +			     int __unused)
> > > +{
> > > +	return cpus_have_const_cap(ARM64_HAS_GENERIC_AUTH_ARCH) ||
> > > +	       cpus_have_const_cap(ARM64_HAS_GENERIC_AUTH_IMP_DEF);
> > > +}
> > 
> > Do these rely on the order in which the entries are listed in the
> > arm64_features[] array? It looks like we do the same for PAN_NOT_UAO but
> > that's pretty fragile.
> 
> Yes, it surely depends on the order in which they are listed.
> 
> > I'd prefer if we invoked the
> > cpu_hwcaps_ptrs[ARM64_HAS_ADDRESS_AUTH]->matches() directly here, maybe
> 
> Yes, calling the matches(cap, SYSTEM_SCOPE), that should work and is much
> better.
> 
> > hidden behind a helper (I couldn't find one at a quick look).
> > 
> 
> There are no helpers for this operation to  do it on a SYSTEM_SCOPE
> and this is only needed for caps dependent on the other caps.
> 
> May be we could hide the conversion of the number to "cap" as:
> 
> static inline struct arm64_cpu_capabilities *cpu_cap_from_number(int n)
> {
> 	if (n < ARM64_NCAPS)
> 		return cpu_hwcaps_ptr[n];
> 	return NULL;
> }
> 
> And use this for "this_cpu_has_cap()" too.

I'm not bothered about the cpu_cap_from_number() part. I was actually
thinking of something like the diff below:

-----------8<-------------------------
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 2595c2886d3f..2ea4c84fcc8a 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -2008,6 +2008,18 @@ bool this_cpu_has_cap(unsigned int n)
 	return false;
 }
 
+static bool system_has_cap(unsigned int n)
+{
+	if (n < ARM64_NCAPS) {
+		const struct arm64_cpu_capabilities *cap = cpu_hwcaps_ptrs[n];
+
+		if (cap)
+			return cap->matches(cap, SCOPE_SYSTEM);
+	}
+
+	return false;
+}
+
 void cpu_set_feature(unsigned int num)
 {
 	WARN_ON(num >= MAX_CPU_FEATURES);
@@ -2081,7 +2093,7 @@ void __init setup_cpu_features(void)
 static bool __maybe_unused
 cpufeature_pan_not_uao(const struct arm64_cpu_capabilities *entry, int __unused)
 {
-	return (cpus_have_const_cap(ARM64_HAS_PAN) && !cpus_have_const_cap(ARM64_HAS_UAO));
+	return system_has_cap(ARM64_HAS_PAN) && !system_has_cap(ARM64_HAS_UAO);
 }
 
 static void __maybe_unused cpu_enable_cnp(struct arm64_cpu_capabilities const *cap)

-- 
Catalin

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