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

[Stefan Bader]

On 26.11.21 18:16, Krzysztof Kozlowski wrote:
> 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].

I am not sure the list is complete (note there is cranky list-derivatives to 
help). The focal name would be focal:linux-azure and at least 
focal:linux-azure-5.11 likely should be there as well. The complete output of

$ cranky list-derivatives impish:linux-azure hirsute:linux-azure focal:linux-azure
* impish/linux-azure
* hirsute/linux-azure
* focal/linux-azure
* focal/linux-azure-5.11
* focal/linux-azure-cvm
* focal/linux-azure-fde
* bionic/linux-azure-5.4

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


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