On Wed, Dec 02, 2020 at 02:29:31PM +0000, Alexandru Elisei wrote:

On 11/19/20 4:58 PM, James Morse wrote:

quoted

On 27/10/2020 17:26, Alexandru Elisei wrote:

quoted

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.

Hmm, so are you saying that with this patch applied, a machine where KVM
isn't even being used can no longer late-online CPUs without SPE if the boot
CPUs had it? If so, then I don't think that's acceptable, unfortunately.

As James points out, the current driver is very careful to support
big.LITTLE misconfigurations and I don't see why KVM support should change
that.

Will

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