[PATCH v2] cpufreq/amd-pstate: Bail out early if !X86_FEATURE_HW_PSTATE

[Rong Zhang]

When booting a VM that simulates or passes-through a relatively new CPU
model, these warning messages are printed to kmsg:

    amd_pstate: The CPPC feature is supported but currently disabled by the BIOS.
    Please enable it if your BIOS has the CPPC option.
    amd_pstate: the _CPC object is not present in SBIOS or ACPI disabled

Technically the check is not wrong and acts as a safetynet that prevents
the driver from being registered incorrectly, but the warning messages
are noisy and incorrect, as the CPPC feature is neither supported in a
VM nor disabled by the VM's BIOS.

The CPPC feature is disabled by the hypervisor as it makes no sense to
expose them to guests. It creates a paradox that the same kernel both
disables CPPC as a hypervisor (KVM) and warns about it being disabled
as a guest.

X86_FEATURE_HW_PSTATE indicates if the platform supports frequency
scaling or not. It is disabled on virtualized platforms (namely, KVM and
Hyper-V have been reported to do so) and is always set on physical
platforms that support frequency scaling.

Check for X86_FEATURE_HW_PSTATE as a prerequisite and bail out early if
it's unsupported.

Fixes: cb817ec6673b ("cpufreq: amd-pstate: show CPPC debug message if CPPC is not supported")
Signed-off-by: Rong Zhang <i@xxxxxxxx>
---
Changes in v2:
- Check for X86_FEATURE_HW_PSTATE instead of X86_FEATURE_HYPERVISOR
  (thanks K Prateek Nayak)
- Remove the check against Xen dom0, as it doesn't need the amd-pstate
  driver (thanks Jason Andryuk)
- Reword comments and the commit message
- Remove Gautham R. Shenoy from the To list due to email bounces
- Link to v1: https://patch.msgid.link/20260716-amd-pstate-vm-v1-1-2ac97d3cf6e7@xxxxxxxx
---
 drivers/cpufreq/amd-pstate.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
index a74a4cf99d22..877b4b30ecd0 100644
--- a/drivers/cpufreq/amd-pstate.c
+++ b/drivers/cpufreq/amd-pstate.c
@@ -2228,6 +2228,13 @@ static int __init amd_pstate_init(void)
 	if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD)
 		return -ENODEV;
 
+	/*
+	 * Do not continue on the following checks and emit bogus warning
+	 * messages if the platform doesn't support frequency scaling at all.
+	 */
+	if (!cpu_feature_enabled(X86_FEATURE_HW_PSTATE))
+		return -ENODEV;
+
 	/* show debug message only if CPPC is not supported */
 	if (!amd_cppc_supported())
 		return -EOPNOTSUPP;

---
base-commit: 94515f3a7d4256a5062176b7d6ed0471938cd51a
change-id: 1cc24037-amd-pstate-vm-d6ab4c959bd3

Thanks,
Rong