Re: [PATCH] cpufreq/amd-pstate: Bail out early in virtual machines and don't warn

[K Prateek Nayak]

Hello Rong,

On 7/15/2026 11:11 PM, Rong Zhang wrote:
> When booting a virtual machine that simulates or passes-through a
> relatively new CPUID, one or two 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 not really helpful, as virtual machines cannot adjust the
> CPU frequency from the very beginning. Moreover, mentioning BIOS here
> makes the messages even more confusing.
> 
> Fix it by bailing out early when running in virtual machines (excluding
> Xen Dom0).
> 
> Fixes: cb817ec6673b ("cpufreq: amd-pstate: show CPPC debug message if CPPC is not supported")
> Signed-off-by: Rong Zhang <i@xxxxxxxx>
> ---
>  drivers/cpufreq/amd-pstate.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
> index a74a4cf99d22..f2115f935318 100644
> --- a/drivers/cpufreq/amd-pstate.c
> +++ b/drivers/cpufreq/amd-pstate.c
> @@ -2228,6 +2228,9 @@ static int __init amd_pstate_init(void)
>  	if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD)
>  		return -ENODEV;
>  
> +	if (cpu_feature_enabled(X86_FEATURE_HYPERVISOR) && !xen_initial_domain())
> +		return -ENODEV;
> +

I think, instead of this, you can just check for X86_FEATURE_HW_PSTATE.

Using check_amd_hwpstate_cpu() from drivers/cpufreq/acpi-cpufreq.c
here should be sufficient to know if the platform supports frequency
scaling or not.

If it does and _CPC object is missing, there is a good chance CPPC is
disabled in BIOS.

KVM doesn't expose X86_FEATURE_HW_PSTATE to guest. I'm not sure if
Xen does.

>  	/* show debug message only if CPPC is not supported */
>  	if (!amd_cppc_supported())
>  		return -EOPNOTSUPP;

-- 
Thanks and Regards,
Prateek