[Bug 2084834] Re: [SRU] cpufreq: intel_pstate: Support Emerald Rapids OOB mode

Patch submitted: https://lists.ubuntu.com/archives/kernel-
team/2024-October/154680.html

** Changed in: linux (Ubuntu Noble)
Status: New => In Progress

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Title:
[SRU] cpufreq: intel_pstate: Support Emerald Rapids OOB mode

Status in linux package in Ubuntu:
New
Status in linux source package in Noble:
In Progress

Bug description:
This is a public version of: https://bugs.launchpad.net/bugs/2078894

[Impact]

In Emerald Rapids VMs with OOB P-states enabled, a stack trace is
printed during boot on 6.8 based kernels. The stack traces look like:

[ 1.206658] intel_pstate: Intel P-state driver initializing
[ 1.207453] unchecked MSR access error: WRMSR to 0x199 (tried to write 0x0000000000000800) at rIP: 0xffffffffb94c3b24 (native_write_msr+0x4/0x40)
[ 1.208422] Call Trace:
[ 1.208422] <TASK>
[ 1.208422] ? show_stack_regs+0x23/0x40
[ 1.208422] ? ex_handler_msr+0x10a/0x180
[ 1.208422] ? fixup_exception+0x183/0x390
[ 1.208422] ? gp_try_fixup_and_notify+0x23/0xc0
[ 1.208422] ? exc_general_protection+0x15e/0x480
[ 1.208422] ? asm_exc_general_protection+0x27/0x30
[ 1.208422] ? __pfx___wrmsr_on_cpu+0x10/0x10
[ 1.208422] ? native_write_msr+0x4/0x40
[ 1.208422] ? __wrmsr_on_cpu+0x4b/0x90
[ 1.208422] ? __pfx___rdmsr_on_cpu+0x10/0x10
[ 1.208422] ? __pfx___wrmsr_on_cpu+0x10/0x10
[ 1.208422] generic_exec_single+0x7e/0x120
[ 1.208422] smp_call_function_single+0x103/0x140
[ 1.208422] ? __pfx___wrmsr_on_cpu+0x10/0x10
[ 1.208422] wrmsrl_on_cpu+0x57/0x80
[ 1.208422] intel_pstate_set_pstate+0x3e/0x80
[ 1.208422] intel_pstate_get_cpu_pstates.constprop.0+0xd7/0x190
[ 1.208422] intel_pstate_init_cpu+0x3f/0x140
[ 1.208422] intel_cpufreq_cpu_init+0x44/0x270
[ 1.208422] ? freq_qos_add_notifier+0x45/0x80
[ 1.208422] cpufreq_online+0x444/0xb80
[ 1.208422] cpufreq_add_dev+0x99/0xd0
[ 1.208422] subsys_interface_register+0x11c/0x140
[ 1.208422] cpufreq_register_driver+0x1b5/0x330
[ 1.208422] intel_pstate_register_driver+0x48/0xd0
[ 1.208422] intel_pstate_init+0x25c/0x810
[ 1.208422] ? __pfx_intel_pstate_init+0x10/0x10
[ 1.208422] do_one_initcall+0x5b/0x310
[ 1.208422] do_initcalls+0x104/0x210
[ 1.208422] ? __pfx_kernel_init+0x10/0x10
[ 1.208422] kernel_init_freeable+0x134/0x1f0
[ 1.208422] kernel_init+0x1b/0x200
[ 1.208422] ret_from_fork+0x44/0x70
[ 1.208422] ? __pfx_kernel_init+0x10/0x10
[ 1.208422] ret_from_fork_asm+0x1b/0x30
[ 1.208422] </TASK>

[Fix]
Cherry pick of 7e1c3f584ee7 - cpufreq: intel_pstate: Support Emerald Rapids OOB mode
Upstream since v6.11.

Very simple change as the OOB identifying bits are same as for the prior
generation CPUs like Sapphire Rapids servers, so also add Emerald Rapids to the
intel_pstate_cpu_oob_ids[] list.

[Test Plan]
Set HW PM OOB mode in BIOS power management config.
Make sure that no cpufreq drivers are loaded. You can verify that
/sys/devices/system/cpu/cpu0/cpufreq/scaling_driver is not present or any cpufreq interface.

It's also already part of linux-aws:
https://bugs.launchpad.net/bugs/2078894

[Where problems could occur]
cpufreq regression.

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