ACK/Cmnt: [PATCH 0/4][I/linux-azure H/linux-azure F/linuz-azure-5.4 B/linux-azure-4.15] add Icelake servers support in no-HWP mode to cpufreq/intel_pstate driver

[Krzysztof Kozlowski]

On 26/11/2021 17:13, Bartlomiej Zolnierkiewicz wrote:
> BugLink: https://bugs.launchpad.net/bugs/1952234
> 
> SRU Justification
> 
> [Impact]
> 
> Starting with the AH2020 Azure host build, Hyper-V is virtualizing some registers that provide information about the CPU frequency. The registers are read-only in a guest VM, so the guest can see the frequency, but cannot make any modifications.
> 
> This feature also requires that the VM Configuration Version be 9.2 or later, which means it needs to be a new VM type, such as the just introduced Dv5/Ev5 series, and the new M832v2 VMs.
> 
> Within the Linux VM, the presence of the feature is indicated by the “aperfmperf” flag in the “lscpu” flags output (or in the flags field in /proc/cpuinfo).
> 
> It turns out there is a Linux kernel limitation when running on the new Intel IceLake processors used for the Dv5/Ev5 series. Upstream commit fbdc21e9b038 was added to provide IceLake support in the 5.14 kernel.
> 
> Microsoft has asked to backport fbdc21e9b038 commit to all supported kernels.
> 
> [Test Plan]
> 
> Run Intel IceLake based VM and check the "aperfmperf" flag in the "lscpu" flags output.
> 
> Without the patch the intel_pstate directory is missing from /sys/devices/system/cpu/ and /sys/devices/system/cpu/cpufreq/ is empty.
> 
> [Where problems could occur]
> 
> * intel_pstate driver is always used on Intel IceLake based VMs without checking for presence of "aperfmperf" CPU flag.
> 
> * In earlier (5.4 and 4.15) linux-azure kernels when intel_pstate driver is used it is in "active" mode instead of "passive" one (as reported by "cat /sys/devices/system/cpu/intel_pstate/status", also "cat /sys/devices/system/cpu/cpufreq/policy0/scaling_driver" returns "intel_pstate" instead of "intel_cpufreq" which is the expected behavior when in "active" mode).
> 
> If a consistent behavior across all kernel versions is desired commit 33aa46f252c7 ("cpufreq: intel_pstate: Use passive mode by default without HWP") from the upstream should probably also be backported.
> 
> * /sys/devices/system/cpu/cpufreq/policy*/scaling_{min,max}_freq files can be modified and the values reported by kernel will no longer match the values used by hardware.
> 
> [Other Info]
> 
> None.
> 

Subject should have [SRU] prefix, sometimes combined with series ([SRU
F/azure]. Hirsute and Impish patches look the same so you could just
name them [I/H / linux-azure].


Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski at canonical.com>

Best regards,
Krzysztof