Re: [PATCH] cpufreq: intel_pstate: Unchecked MSR aceess in legacy mode

Rafael J. Wysocki
2025-04-30T14:29:22Z

On Tue, Apr 29, 2025 at 11:07 PM Srinivas Pandruvada
<srinivas.pandruvada@linux.intel.com> wrote:
>
> 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@linux.intel.com>
> Cc: stable@vger.kernel.org
> ---
> 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 f41ed0b9e610..ba9bf06f1c77 100644
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -598,6 +598,9 @@ static bool turbo_is_disabled(void)
> {
> u64 misc_en;
>
> + if (!cpu_feature_enabled(X86_FEATURE_IDA))
> + return true;
> +
> rdmsrl(MSR_IA32_MISC_ENABLE, misc_en);
>
> return !!(misc_en & MSR_IA32_MISC_ENABLE_TURBO_DISABLE);
> --

Applied as a fix for 6.15-rc, thanks!