[RFC PATCH v3 04/16] arm64: Introduce CPU SPE feature

[Alexandru Elisei]

Hi James,

On 11/19/20 4:58 PM, James Morse wrote:
> Hi Alex,
>
> On 27/10/2020 17:26, Alexandru Elisei wrote:
>> Detect Statistical Profiling Extension (SPE) support using the cpufeatures
>> framework. The presence of SPE is reported via the ARM64_SPE capability.
>>
>> The feature will be necessary for emulating SPE in KVM, because KVM needs
>> that all CPUs have SPE hardware to avoid scheduling a VCPU on a CPU without
>> support. For this reason, the feature type ARM64_CPUCAP_SYSTEM_FEATURE has
>> been selected to disallow hotplugging a CPU which doesn't support SPE.
> Can you mention the existing driver in the commit message? Surprisingly it doesn't use
> cpufeature at all. It looks like arm_spe_pmu_dev_init() goes out of its way to support
> mismatched systems. (otherwise the significance of the new behaviour isn't clear!)
>
> I read it as: the host is fine with mismatched systems, and the existing drivers supports
> this. But KVM is not. After this patch you can't make the system mismatched 'late'.

That was exactly my intention. Certainly, I'll try to make the commit message
clearer by mentioning the SPE driver.

Thanks,
Alex