Re: [RFC PATCH 0/2] cpufreq/amd-pstate: Fixes for "Requested CPU Min frequency" support

[K Prateek Nayak]

Hello Mario,

Thank you for reviewing the series.

On 7/17/2026 1:20 AM, Mario Limonciello wrote:
> 
> 
> On 7/15/26 02:48, K Prateek Nayak wrote:
>> It was noted during internal testing that amd-pstate active mode with
>> performance governor always sets the min_perf to nominal_perf despite
>> BIOS having supplied bios_min_perf as the preferred idling perf.
>>
>> Users who set the "Requested CPU Min frequency" from BIOS are known to
>> have profiled their workload at different operating frequency to know
>> the best configuration and amd-pstate should use the same as the lower
>> limit when configured.
>>
>> While testing the fix for above, it was noted that kexec fails to
>> persist bios_min_freq even when both, the old and new kernel are aware
>> of bios_min_perf.
>>
>> The suspend, offlining paths switched to persisting the CPPC_REQ MSR
>> state at the time of suspend / offlining with only min_perf being reset
>> to bios_min_perf.
>>
>> msr_init() path only accepts the configured min_perf as bios_min_perf
>> when it finds rest of the bits in CPPC_REQ MSR to be 0. This is
>> intentional to prevent stale value of last CPPC_REQ from kernels that
>> are not aware of bios_min_perf to be mistakenly interpreted as the
>> bios_min_perf during kexec boot.
> 
> Is it a real valid case we need to worry about for someone kexec'ing between kernels that are aware of this vs not aware of it?

Since there are defensive measure in place I thought it might have
been a concern but as you said, I doubt anyone is running one of
those flavors and kexecing back and forth that often.


> During kernel development; sure this might happen.  But I would think once this is available in mainline any of the distro kernels will have picked this up and people will start with a distro kernel with support, or they'll start with a distro kernel without and upgrade to one with.
> 
> I can't imagine a case people will go the other way.

Ack! So I'm thinking of changing Patch 2 to:

diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
index 27cfacd283be..707e1485eb1c 100644
--- a/drivers/cpufreq/amd-pstate.c
+++ b/drivers/cpufreq/amd-pstate.c
@@ -462,7 +462,6 @@ static int msr_init_perf(struct amd_cpudata *cpudata)
 {
 	union perf_cached perf = READ_ONCE(cpudata->perf);
 	u64 cap1, numerator, cppc_req;
-	u8 min_perf;
 
 	int ret = rdmsrq_safe_on_cpu(cpudata->cpu, MSR_AMD_CPPC_CAP1,
 				     &cap1);
@@ -478,16 +477,6 @@ static int msr_init_perf(struct amd_cpudata *cpudata)
 		return ret;
 
 	WRITE_ONCE(cpudata->cppc_req_cached, cppc_req);
-	min_perf = FIELD_GET(AMD_CPPC_MIN_PERF_MASK, cppc_req);
-
-	/*
-	 * Clear out the min_perf part to check if the rest of the MSR is 0, if yes, this is an
-	 * indication that the min_perf value is the one specified through the BIOS option
-	 */
-	cppc_req &= ~(AMD_CPPC_MIN_PERF_MASK);
-
-	if (!cppc_req)
-		perf.bios_min_perf = min_perf;
 
 	perf.highest_perf = numerator;
 	perf.max_limit_perf = numerator;
@@ -495,6 +484,7 @@ static int msr_init_perf(struct amd_cpudata *cpudata)
 	perf.nominal_perf = FIELD_GET(AMD_CPPC_NOMINAL_PERF_MASK, cap1);
 	perf.lowest_nonlinear_perf = FIELD_GET(AMD_CPPC_LOWNONLIN_PERF_MASK, cap1);
 	perf.lowest_perf = FIELD_GET(AMD_CPPC_LOWEST_PERF_MASK, cap1);
+	perf.bios_min_perf = FIELD_GET(AMD_CPPC_MIN_PERF_MASK, cppc_req);
 	WRITE_ONCE(cpudata->perf, perf);
 	WRITE_ONCE(cpudata->prefcore_ranking, FIELD_GET(AMD_CPPC_HIGHEST_PERF_MASK, cap1));
 	WRITE_ONCE(cpudata->floor_perf_cnt, FIELD_GET(AMD_CPPC_FLOOR_PERF_CNT_MASK, cap1));
@@ -1046,6 +1036,13 @@ static int amd_pstate_init_freq(struct amd_cpudata *cpudata)
 		return -EINVAL;
 	}
 
+	if (perf.bios_min_perf) {
+		u32 bios_min_freq = perf_to_freq(perf, cpudata->nominal_freq, perf.bios_min_perf);
+
+		pr_debug("Found Requested CPU Min Frequency of %u on CPU%d\n",
+			 bios_min_freq, cpudata->cpu);
+	}
+
 	return 0;
 }
 
---

It gets rid of that defensive check and leaves enough breadcrumbs to
debug any issues that might arise during a kexec. Thoughts?

-- 
Thanks and Regards,
Prateek