On Wed, Dec 02, 2020 at 02:29:31PM +0000, Alexandru Elisei wrote: > On 11/19/20 4:58 PM, James Morse wrote: > > 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. 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 _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
