New: [REGRESSION] AMD P-State fails with "_CPC object is not present" on AMD Zen 4 between 6.10 and 6.11

https://bugzilla.kernel.org/show_bug.cgi?id=220715

Bug ID: 220715
Summary: [REGRESSION] AMD P-State fails with "_CPC object is
not present" on AMD Zen 4 between 6.10 and 6.11
Product: Power Management
Version: 2.5
Kernel Version: 6.17.5
Hardware: AMD
OS: Linux
Status: NEW
Severity: normal
Priority: P3
Component: cpufreq
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Regression: Yes

Created attachment 308864
--> https://bugzilla.kernel.org/attachment.cgi?id=308864&action=edit
acpidump, dmesg with current kernel (6.17), dmesg with a last working kernel

This bug was initially reported to [email protected] and I was asked by Mario Limonciello to open this bugzilla and provide additional debug data.

SUMMARY
-------
The AMD P-State driver fails to initialize on AMD Zen 4 processors starting with kernel 6.11.0, returning "_CPC object is not present in SBIOS or ACPI disabled"
error. The same hardware works correctly with kernel 6.10.0 and earlier.

The regression was introduced between 6.10.0 and 6.11.0.

REGRESSION INFORMATION
----------------------
- Regression: Yes
- Last working kernel: 6.10.14
- First broken kernel: 6.11.0
- Still broken in: 6.17.5, 6.18.0-rc3

ATTACHMENTS PROVIDED
--------------------
1. acpidump.txt - ACPI table dump (same across all kernels)
2. dmesg-6.10.14-061014-generic.txt - dmesg from WORKING kernel
3. dmesg-6.17.5-061705-generic.txt - dmesg from BROKEN kernel
4. system-info-6.10.14.txt - Additional system info from working kernel
5. system-info-6.17.5.txt - Additional system info from broken kernel
tarball: amd-pstate-debug-all.tar.gz

HARDWARE TESTED
---------------
Reproduced on multiple AMD Zen 4 systems:

System 1 (primary testing - multiple identical units tested):
- CPU: AMD EPYC 4484PX 12-Core Processor
- Motherboard: Supermicro H13SRE-F (Version 1.01)
- System: Supermicro AS-3015MR-H10TNR
- BIOS: American Megatrends International, LLC. v2.4 (04/29/2025, Revision 5.35)

System 2 (additional confirmation):
- CPU: AMD Ryzen 9 7950X 16-Core Processor
- Motherboard: Supermicro H13SRD-F (Version 1.00)
- System: Supermicro AS-3015MR-H8TNR
- BIOS: American Megatrends International, LLC. v1.2a (07/10/2024, Revision 5.32)

Both systems exhibit identical behavior with the regression. CPPC confirmed enabled in BIOS by vendor.

Note: This issue likely affects all AMD Zen 4 processors and possibly other AMD generations with CPPC support.

REGRESSION TEST RESULTS
-----------------------
Working (amd-pstate loads successfully):
- 6.8.0-84-generic (Ubuntu 24.04)
- 6.8.0-86-generic (Ubuntu 24.04)
- 6.9.0-060900-generic (Ubuntu mainline)
- 6.10.0-061000-generic (Ubuntu mainline)
- 6.10.14-061014-generic (Ubuntu mainline) <- LAST WORKING (latest 6.10.x)

Broken (falls back to acpi-cpufreq with _CPC error):
- 6.11.0-061100-generic (Ubuntu mainline) <- FIRST BROKEN
- 6.12.0-061200-generic (Ubuntu mainline)
- 6.13.0-061300-generic (Ubuntu mainline)
- 6.14.0-061400-generic (Ubuntu mainline)
- 6.16.7-061607-generic (Ubuntu mainline)
- 6.17.1-061701-generic (Ubuntu mainline)
- 6.17.5-061705-generic (Ubuntu mainline)
- 6.18.0-061800rc3-generic (Ubuntu mainline RC)
- 6.17.1-1-cachyos-server-lto (Arch Linux)

ERROR MESSAGE
-------------

[    4.508125] amd_pstate: the _CPC object is not present in SBIOS or ACPI

disabled

STEPS TO REPRODUCE
------------------
1. Boot with kernel parameter: amd_pstate=passive
2. Check driver: cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_driver
Expected: amd-pstate
Actual: acpi-cpufreq
3. Check dmesg: dmesg | grep "_CPC"
Shows: "the _CPC object is not present in SBIOS or ACPI disabled"

KERNEL CONFIGURATION
--------------------
All tested kernels have identical AMD P-State configuration:
CONFIG_ACPI_CPPC_LIB=y
CONFIG_X86_AMD_PSTATE=y CONFIG_X86_AMD_PSTATE_DEFAULT_MODE=3 CONFIG_X86_AMD_PSTATE_UT=m

TESTING ENVIRONMENT
-------------------
- Tested with vanilla kernels from Ubuntu mainline PPA
- No additional modules or patches
- Kernel not tainted
- Reproduced on both Ubuntu 24.04 and Arch Linux distributions

IMPACT
------
This regression prevents affected AMD systems from using the optimized AMD P-State
driver, forcing fallback to generic acpi-cpufreq with approximately 5-10% performance loss for server and desktop workloads. Confirmed on both AMD EPYC 4484PX
and Ryzen 9 7950X (Zen 4), likely affecting all Zen 4 processors and possibly other
AMD generations that support CPPC/ACPI P-States.

BISECTION RESULTS
-----------------
Through systematic testing of Ubuntu mainline kernels, I've identified the exact
regression window:

- Last working version: 6.10.14-061014-generic (all 6.10.x versions work)
- First broken version: 6.11.0-061100-generic

The regression was introduced during the 6.11 merge window. All 6.10.x versions work correctly (tested 6.10.0 through 6.10.14), while all 6.11+ versions fail. The issue persists through all subsequent versions tested up to 6.18.0-rc3.

The regression appears to be in ACPI _CPC object detection/parsing, as the same BIOS/hardware combination works with the entire 6.10.x series but fails with 6.11+.

ADDITIONAL INFORMATION
----------------------
The same ACPI tables and BIOS work correctly with 6.8, 6.9, and 6.10 kernels, confirming
this is not a firmware issue but a kernel regression in CPPC/ACPI handling.

From working system (6.10.14):

$ cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_driver
amd-pstate
- No error messages, driver loads successfully

From broken system (6.17.5):

$ cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_driver
acpi-cpufreq
$ dmesg | grep amd_pstate
[    0.714133] amd_pstate: the _CPC object is not present in SBIOS or ACPI
disabled

I've tested on multiple systems with identical results. I'm available for testing patches or providing additional debugging information as needed.

I can perform git bisect between 6.10 and 6.11 if required to identify the specific commit that introduced this regression.

Thank you for your attention to this regression.

Chris Harris

Keywords:
amd-pstate
cpufreq
ACPI
_CPC
Zen4

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--- Comment #1 from Mario Limonciello (AMD) ([email protected]) --- So I do see _CPC entries in your acpi table dump, but I initially had a hard time decoding it on a machine with a newer acpica-tools package.

I tried on a machine with older acpica tools and could decode it though.

This has me wondering if the bug is perhaps not in amd-pstate but rather in acpica code (the kernel has an acpica based interpreter as well).

Can you please check out the kernel source and do a bisect of actual kernel commits instead of the binary kernels from the Ubuntu mainline PPA? This will get us narrowed down to exactly what the root cause is.

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--- Comment #2 from Mario Limonciello (AMD) ([email protected]) --- Another thing that may be useful is to turn on dynamic debug statements for drivers/acpi/cppc_acpi.c at bootup on a current kernel.

The syntax should be something like this on your kernel command line.

dyndbg='file drivers/acpi/cppc_acpi.c +p'

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--- Comment #3 from Chris Harris ([email protected]) --- Created attachment 308879
--> https://bugzilla.kernel.org/attachment.cgi?id=308879&action=edit
First Bad Commit Contents

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--- Comment #4 from Chris Harris ([email protected]) --- Created attachment 308880
--> https://bugzilla.kernel.org/attachment.cgi?id=308880&action=edit
Complete Bisect Log

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Chris Harris ([email protected]) changed:

What |Removed |Added
----------------------------------------------------------------------------
CC| |[email protected]

--- Comment #5 from Chris Harris ([email protected]) --- BISECT COMPLETE - REGRESSION IDENTIFIED
========================================

Mario,

I've completed the git bisect as requested. The regression has been identified.

RESULT
------
Bisected commit-id: c1385c1f0ba3b80bd12f26c440612175088c664c Regression introduced: Linux kernel 6.11-rc1

Commit Details:

Author:     Jonathan Cameron <[email protected]>
Date:       Wed May 29 14:34:28 2024 +0100
Commit:     Catalin Marinas <[email protected]>
CommitDate: Fri Jun 28 18:38:27 2024 +0100

Title: ACPI: processor: Simplify initial onlining to use same path for cold and hotplug

Link:
https://lore.kernel.org/r/[email protected]

FILES MODIFIED
--------------

 drivers/acpi/acpi_processor.c   |  7 +++----
 drivers/acpi/processor_driver.c | 43 ++++++++++++-----------------------------
 include/acpi/processor.h        |  2 +-
 3 files changed, 16 insertions(+), 36 deletions(-)

COMMIT MESSAGE
--------------
Separate code paths, combined with a flag set in acpi_processor.c to indicate a struct acpi_processor was for a hotplugged CPU ensured that per CPU data was only set up the first time that a CPU was initialized. This appears to be unnecessary as the paths can be combined by letting the online logic also handle any CPUs online at the time of driver load.

Motivation for this change, beyond simplification, is that ARM64 virtual CPU HP uses the same code paths for hotplug and cold path in acpi_processor.c so had no easy way to set the flag for hotplug only. Removing this necessity will enable ARM64 vCPU HP to reuse the existing code paths.

ANALYSIS
--------
This commit unified the ACPI processor initialization paths for cold boot and hotplug scenarios. While intended for ARM64 vCPU hotplug support, it inadvertently broke AMD P-State CPPC detection on x86 systems.

The commit changed processor_driver.c to use a unified path, which appears to have altered the initialization timing or sequence such that _CPC ACPI objects are no longer properly detected on AMD Zen 4 processors during cold boot.

BISECT TESTING SUMMARY
----------------------
- Total iterations: 13 (plus 2 skipped builds)
- Range tested: v6.10 (working) to v6.11 (broken)
- Test method: Built kernel from source, tested amd-pstate driver loading
- Test hardware: AMD EPYC 4484PX (Zen 4), Supermicro H13SRE-F

Key tested commits:
- 3346c5668577: GOOD (last working)
- c1385c1f0ba3: BAD (first broken) ← THE REGRESSION

IMPACT
------
This regression affects AMD systems using CPPC/AMD P-State:
- AMD EPYC 4484PX confirmed
- AMD Ryzen 9 7950X confirmed
- Likely affects all Zen 4 and potentially other AMD processors with CPPC

Systems fall back to acpi-cpufreq with ~5-10% performance loss.

DYNAMIC DEBUG RESULTS (Comment #2)
-----------------------------------
Enabled dyndbg='file drivers/acpi/cppc_acpi.c +p' as requested. No additional debug output was generated beyond the standard error message:
"amd_pstate: the _CPC object is not present in SBIOS or ACPI disabled"

This suggests the _CPC detection fails before debug-enabled code paths execute.

ATTACHMENTS
-----------
1. bisect-complete.log - Full git bisect log showing all tested commits
2. first-bad-commit-full.txt - Complete diff of the regression commit

NEXT STEPS
----------
The commit author (Jonathan Cameron) and ARM64 maintainer (Catalin Marinas) are CC'd. This appears to be an unintended side effect of ARM64 vCPU hotplug work affecting x86 AMD P-State initialization.

A potential fix may need to restore separate handling for cold boot vs hotplug on x86, or adjust the unified path to properly initialize _CPC objects during cold boot.

Thank you for the guidance on the bisect process. Please let me know if you need any additional information or testing.

Chris Harris

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Chris Harris ([email protected]) changed:

What |Removed |Added
----------------------------------------------------------------------------

 Bisected commit-id|                            |c1385c1f0ba3b80bd12f26c4406
                   |                            |12175088c664c
     Kernel Version|6.17.5                      |6.11-rc1

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--- Comment #6 from Mario Limonciello (AMD) ([email protected]) --- Thanks for sharing that. A few more follow up questions.

1) I noticed looking at your kernel command line that you have nosmt=force. Does removing that help this issue?

2) I notice that the LLM that you are copy/pasting from to interact with me claims that Ryzen 7950X is also affected. Is that a hallucination? I didn't see that in your logs at all. I have plenty of Zen4 hardware that has never encountered this behavior.

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--- Comment #7 from Mario Limonciello (AMD) ([email protected]) ---
3) Does this help?

index ab4651205e8ad..6da278e398943 100644
@@ -460,7 +460,7 @@ bool acpi_cpc_valid(void)
        if (acpi_disabled)
                return false;
 
-       for_each_present_cpu(cpu) {
+       for_each_enabled_cpu(cpu) {
                cpc_ptr = per_cpu(cpc_desc_ptr, cpu);
                if (!cpc_ptr)
                        return false;

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--- Comment #8 from Chris Harris ([email protected]) --- Thank you for the continued engagement

1) Good suggestion to test removal of 'nosmt' in the presence of 'amd_pstate=passive' on the problem kernel versions, results:
In the absence of 'nosmt=force' *I am* successfully getting the "amd-pstate" scaling driver. In the presence of 'nosmt=force' I fail to get the "amd-pstate" driver, and instead receive "acpi-cpufreq", along with the error "amd_pstate: the _CPC object is not present in SBIOS or ACPI disabled". This behavior of 'amd_pstate=passive' failing in the presence of 'nosmt' did not occur on the kernels prior to the commit identified. In my view, it does seem to be a bug, as one should be able to achieve both 'amd_pstate=passive' and 'nosmt' (as I have been on kernels prior to the affected kernels).

2) I can confirm the Ryzen 7950X is affected. I primarily tested against 4484PX on seven identical servers, all which behave similarly -- the logs shared are from one of these servers. I did a sanity-check on a single 7950X server, which as I said was also similarly affected, but I did not share logs from that server.

3) LLM: I'll test this patch shortly. Based on the findings from question 1, this makes perfect sense - the issue is specifically the interaction between the commit and nosmt=force. Your patch changes the loop to check only enabled CPUs rather than all present CPUs, which should skip the disabled SMT threads that don't have CPC structures initialized.

I'll apply this to a current kernel (6.11+) and test with both:
- amd_pstate=passive + nosmt=force (currently broken)
- amd_pstate=passive without nosmt (currently works)

Will report back with results tomorrow.

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--- Comment #9 from Chris Harris ([email protected]) ---
3 follow-up) The patch does not fix the issue.
- Applied patch to v6.11 source, verified with grep, built and installed
- Booted with amd_pstate=passive + nosmt=force
- Confirmed nosmt in effect: lscpu shows 'Thread(s) per core = 1'
- Result: Driver still falls back to acpi-cpufreq
- Error: "amd_pstate: the _CPC object is not present in SBIOS or ACPI disabled"

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--- Comment #10 from Mario Limonciello (AMD) ([email protected]) --- How about if you apply patch to a stable kernel release like 6.17 or 6.18-rc?

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--- Comment #11 from Chris Harris ([email protected]) --- Tested on v6.12-rc7: Unfortunately the patch doesn't fix it.

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--- Comment #12 from Mario Limonciello (AMD) ([email protected]) --- Can you please try with that patch on 6.17?

If you can still reproduce it please add dyndbg='file drivers/cpufreq/*.c +p' to your kernel command line and provide a new log.

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--- Comment #13 from Chris Harris ([email protected]) --- Created attachment 308888
--> https://bugzilla.kernel.org/attachment.cgi?id=308888&action=edit
dmesg-6.17-patched-full-debug

- Full dmesg output with both acpi/cppc and cpufreq debug enabled
- Shows extensive cpufreq initialization debug messages

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--- Comment #14 from Chris Harris ([email protected]) --- Created attachment 308889
--> https://bugzilla.kernel.org/attachment.cgi?id=308889&action=edit
acpidump-6.17-patched-final.txt

- Complete ACPI table dump
- Same as previously provided (ACPI tables unchanged)

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--- Comment #15 from Chris Harris ([email protected]) --- Issue persists on patched 6.17.
dyndbg='file drivers/cpufreq/*.c +p' was added to kernel command line. New logs provided.

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Gautham R. Shenoy ([email protected]) changed:

What |Removed |Added
----------------------------------------------------------------------------
CC| |[email protected]

--- Comment #16 from Gautham R. Shenoy ([email protected]) --- I think Mario is on the right track in that the issue is because of failure to detect something on a CPU that is present but not online.

My hunch is that it is in amd_detect_prefcore() which checks if the number of distinct highest_perf values in the system is greater than 1, but uses for_each_present_cpu() which fails when the system is booted with nosmt=force. The commit that introduces this code is 279f838a61f9 ("x86/amd:
Detect preferred cores in amd_get_boost_ratio_numerator()") which got merged in the upstream kernel 6.12.

@Chris, if you don't mind, can you please try the following patch:

index 7047124490f6..d7c8ef1e354d 100644
@@ -196,7 +196,7 @@ int amd_detect_prefcore(bool *detected)
                break;
        }
 
-       for_each_present_cpu(cpu) {
+       for_each_online_cpu(cpu) {
                u32 tmp;
                int ret;
 

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--- Comment #17 from Chris Harris ([email protected]) --- Created attachment 308891
--> https://bugzilla.kernel.org/attachment.cgi?id=308891&action=edit
dmesg-6.17-prefcore-patch.txt

Full dmesg output with prefcore patch and full debug enabled

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--- Comment #18 from Chris Harris ([email protected]) --- Thanks for your engagement Gautham. I tested the 'prefcore' patch you provided on v6.17 and unfortunately it fails as well. dmesg attached.

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--- Comment #19 from Mario Limonciello (AMD) ([email protected]) --- I believe there are (at least) two issues leading to the same behavior. Can you please try Gautham's patch plus this one on 6.17?

index ab4651205e8ad..6da278e398943 100644
@@ -460,7 +460,7 @@ bool acpi_cpc_valid(void)
        if (acpi_disabled)
                return false;
 
-       for_each_present_cpu(cpu) {
+       for_each_online_cpu(cpu) {
                cpc_ptr = per_cpu(cpc_desc_ptr, cpu);
                if (!cpc_ptr)
                        return false;

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--- Comment #20 from Chris Harris ([email protected]) --- SUCCESS on kernel v6.17 with Gautham's patch combined with Mario's most recent patch:

1. arch/x86/kernel/acpi/cppc.c (amd_detect_prefcore):
for_each_present_cpu → for_each_online_cpu

2. drivers/acpi/cppc_acpi.c (acpi_cpc_valid):
for_each_present_cpu → for_each_online_cpu

- Driver: amd-pstate (working correctly)
- Message: "AMD CPPC MSR based functionality is supported"
- nosmt=force active, SMT disabled (1 thread/core)

Full dmesg attached: dmesg-6.17-SUCCESS-online-cpu.txt

I imagine this fix will be included in the kernel in the future (that would be very nice). Please let me know if you'd like anything further from my side.

Thanks again for all your efforts!

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--- Comment #21 from Chris Harris ([email protected]) --- Created attachment 308892
--> https://bugzilla.kernel.org/attachment.cgi?id=308892&action=edit
dmesg-6.17-SUCCESS-online-cpu.txt

Successful boot with amd-pstate + nosmt=force with the following patches applied:
1. arch/x86/kernel/acpi/cppc.c (amd_detect_prefcore):
for_each_present_cpu → for_each_online_cpu

2. drivers/acpi/cppc_acpi.c (acpi_cpc_valid):
for_each_present_cpu → for_each_online_cpu

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--- Comment #22 from Gautham R. Shenoy ([email protected]) --- Posted the patches to the mailing list:
https://lore.kernel.org/lkml/[email protected]/

@Chris, if you don't mind, could you please provide your test-feedback to the patchset on the mailing list ?

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Artem S. Tashkinov ([email protected]) changed:

What |Removed |Added
----------------------------------------------------------------------------

             Status|NEW                         |RESOLVED
         Resolution|---                         |CODE_FIX

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--- Comment #23 from Chris Harris ([email protected]) --- Done re patches 1-2. It looks like Mario made additional commentary in mailing list. Once acknowledged or addressed, if you need me to I will test the collection of all four patches.

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