[RFC PATCH 4/7] arm64: cpufeature: Enable PBHA bits for stage1

[James Morse]

Hi Marc,

On 16/10/2021 14:50, Marc Zyngier wrote:
> On Fri, 15 Oct 2021 17:14:13 +0100,
> James Morse <james.morse at arm.com> wrote:
>>
>> If the CPUs support HPDS2, and there is a DT description of PBHA values
>> that only affect performance, enable those bits for both TTBR0 and TTBR1.
>>
>> Enabling PBHA requires the hierarchical-permissions to be disabled.
>> Commit 87143f404f33 ("arm64: mm: use XN table mapping attributes for
>> the linear region") used these, but only as an optimisation.
>>
>> Only the necessary PBHA bits are enabled to reduce the risk of an
>> unsafe bit/value being used by accident.

>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>> index 8694f9dec5e5..548c6f96a878 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c

>> @@ -1676,6 +1679,71 @@ static bool has_hw_dbm(const struct arm64_cpu_capabilities *cap,

>> +static bool plat_can_use_pbha_stage1(const struct arm64_cpu_capabilities *cap,
>> +				     int scope)
>> +{
>> +	u8 val;
>> +	struct device_node *cpus;
>> +	const u8 *perf_only_vals;
>> +	int num_perf_only_vals, i;
>> +
>> +	if (!has_cpuid_feature(cap, scope))
>> +		return false;
>> +
>> +	/*
>> +	 * Calls with scope == SCOPE_LOCAL_CPU need only testing whether this
>> +	 * cpu has the feature. A later 'system' scope call will check for a
>> +	 * firmware description.
>> +	 */
>> +	if (scope == SCOPE_LOCAL_CPU)
>> +		return true;
>> +
>> +	cpus = of_find_node_by_path("/cpus");
>> +	if (!cpus)
>> +		goto done;
>> +
>> +	perf_only_vals = of_get_property(cpus, "arm,pbha-performance-only",
>> +					 &num_perf_only_vals);
>> +	if (!perf_only_vals)
>> +		goto done;
>> +
>> +	/* any listed value is usable at stage 1 */
>> +	for (i = 0 ; i < num_perf_only_vals; i++) {
>> +		val = perf_only_vals[i];
>> +		if (val > 0xf)
>> +			continue;
>> +
>> +		pbha_stage1_enable_bits |= val;
>> +		set_bit(val, &arm64_pbha_perf_only_values);
>> +	}

> Somehow, this would need to be exposed to userspace so that a VMM
> could tell a guest what it can use.

I'm assuming any user is very soc-specific... but it would help the VMM to know.

I guess KVM could return the bitmap as KVM_CAP_PBHA. There is no way to tell the VMM what
the bits do, as that is imp-def...


Thanks,

James