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

[Will Deacon]

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