Re: [PATCH v2] x86/platform: Adjust temperature sensors MSRs

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.02.2026 16:53, Teddy Astie wrote:
> Temperature sensors MSR were previously assumed to be available when "DTS"
> CPUID bit is set. This is not really the case :
>  * MSR_IA32_THERM_STATUS is gated behind ACPI CPUID bit, only DTS-related bits
> of this MSR are gated behind the DTS CPUID
>  * MSR_PACKAGE_THERM_STATUS is gated behind "PTM" CPUID
> 
> Also adjust the MSR_TEMPERATURE_TARGET which is not architectural, but stable
> in practice, and required to be exposed for reliably querying CPU temperature.
> 
> Fixes: 615c9f3f820 ("x86/platform: Expose DTS sensors MSR")
> Reported-by: Tu Dinh <ngoc-tu.dinh@xxxxxxxxxx>
> Signed-off-by: Teddy Astie <teddy.astie@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

> --- a/xen/arch/x86/platform_hypercall.c
> +++ b/xen/arch/x86/platform_hypercall.c
> @@ -89,9 +89,26 @@ static bool msr_read_allowed(unsigned int msr)
>          return cpu_has_srbds_ctrl;
>  
>      case MSR_IA32_THERM_STATUS:
> +        return host_cpu_policy.basic.acpi;
> +
> +    /*
> +     * This MSR is present on most Intel Core-family CPUs since Nehalem but 
> is not an
> +     * architectural MSR. No CPUID bit enumerates this MSR.
> +     *
> +     * This MSR exposes "temperature target" that is needed to compute the 
> CPU
> +     * temperature. The "temperature target" is a model specific value, and 
> this MSR is
> +     * the only known method of getting the one used for the CPU. On some 
> CPU models with
> +     * Intel SST-PP, the "temperature target" can vary over time.
> +     *
> +     * We assume all Intel CPUs with DTS may support this MSR; but reads can 
> fail in case
> +     * the platform doesn't actually support this MSR.
> +     */
>      case MSR_TEMPERATURE_TARGET:
> +        return boot_cpu_data.x86_vendor == X86_VENDOR_INTEL &&
> +            host_cpu_policy.basic.digital_temp_sensor;

Personally I think indentation wants to be three deeper here, but this is one
of the grey areas of our style. I may take the liberty to adjust it while
committing.

Jan