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

[James Morse]

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'.


Thanks,

James