Acked: [SRU][N:linux][PATCH 1/1] cpufreq: intel_pstate: Unchecked MSR aceess in legacy mode

[Aaron Ma]

Acked-by: Aaron Ma <aaron.ma at canonical.com>

On Fri, Dec 19, 2025 at 11:11 PM Thibault Ferrante
<thibault.ferrante at canonical.com> wrote:
>
> From: Srinivas Pandruvada <srinivas.pandruvada at linux.intel.com>
>
> BugLink: https://bugs.launchpad.net/bugs/2130244
>
> When turbo mode is unavailable on a Skylake-X system, executing the
> command:
>
>  # echo 1 > /sys/devices/system/cpu/intel_pstate/no_turbo
>
> results in an unchecked MSR access error:
>
>  WRMSR to 0x199 (attempted to write 0x0000000100001300).
>
> This issue was reproduced on an OEM (Original Equipment Manufacturer)
> system and is not a common problem across all Skylake-X systems.
>
> This error occurs because the MSR 0x199 Turbo Engage Bit (bit 32) is set
> when turbo mode is disabled. The issue arises when intel_pstate fails to
> detect that turbo mode is disabled. Here intel_pstate relies on
> MSR_IA32_MISC_ENABLE bit 38 to determine the status of turbo mode.
> However, on this system, bit 38 is not set even when turbo mode is
> disabled.
>
> According to the Intel Software Developer's Manual (SDM), the BIOS sets
> this bit during platform initialization to enable or disable
> opportunistic processor performance operations. Logically, this bit
> should be set in such cases. However, the SDM also specifies that "OS
> and applications must use CPUID leaf 06H to detect processors with
> opportunistic processor performance operations enabled."
>
> Therefore, in addition to checking MSR_IA32_MISC_ENABLE bit 38, verify
> that CPUID.06H:EAX[1] is 0 to accurately determine if turbo mode is
> disabled.
>
> Fixes: 4521e1a0ce17 ("cpufreq: intel_pstate: Reflect current no_turbo state correctly")
> Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada at linux.intel.com>
> Cc: All applicable <stable at vger.kernel.org>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki at intel.com>
> (cherry picked from commit ac4e04d9e378f5aa826c2406ad7871ae1b6a6fb9)
> Signed-off-by: Thibault Ferrante <thibault.ferrante at canonical.com>
> ---
>  drivers/cpufreq/intel_pstate.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
> index 366f06d8e5f44..cb3c3d40734cd 100644
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -600,6 +600,9 @@ static inline void update_turbo_state(void)
>  {
>         u64 misc_en;
>
> +       if (!cpu_feature_enabled(X86_FEATURE_IDA))
> +               return true;
> +
>         rdmsrl(MSR_IA32_MISC_ENABLE, misc_en);
>         global.turbo_disabled = misc_en & MSR_IA32_MISC_ENABLE_TURBO_DISABLE;
>  }
> --
> 2.51.0
>
>
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