Re: [PATCH] x86: Clear LAM and FRED feature bits

[Xin Li]

> On Jul 22, 2025, at 12:54 AM, Maciej Wieczor-Retman <maciej.wieczor-retman@xxxxxxxxx> wrote:
>
> If LAM (CONFIG_ADDRESS_MASKING) or FRED (CONFIG_X86_FRED) are disabled
> during compile time, they still are enumerated in macros such as
> cpu_has() or this_cpu_has() that use the x86_capability bitmask. The
> features are also visible in /proc/cpuinfo even though they are not
> enabled - which is contrary to what the documentation states about the
> file.

I'm curious how this bug is introduced with my patch set that adds the
AWK script which automatically generates the CPU feature mask header.

> The documentation for x86/cpuinfo.rst also states how to handle features
> disabled at compile time:
>
>    The kernel disabled support for it at compile-time
>    --------------------------------------------------
>
>    For example, if Linear Address Masking (LAM) is not enabled when
>    building (i.e., CONFIG_ADDRESS_MASKING is not selected) the flag
>    "lam" will not show up. Even though the feature will still be
>    detected via CPUID, the kernel disables it by clearing via
>    setup_clear_cpu_cap(X86_FEATURE_LAM).
>
> Clear feature bits if they are present in the DISABLED_MASK.
>
> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@xxxxxxxxx>

And we need a Fixes tag and CC stable.

> ---
> arch/x86/kernel/cpu/common.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 77afca95cced..1c5af795cedc 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1782,6 +1782,16 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
>    if (!pgtable_l5_enabled())
>        setup_clear_cpu_cap(X86_FEATURE_LA57);
>
> +    /*
> +     * If a feature is disabled during compile time clear its feature
> +     * bit to prevent it from showing up in the x86_capability bitmask.
> +     */
> +    if (!cpu_feature_enabled(X86_FEATURE_LAM))
> +        setup_clear_cpu_cap(X86_FEATURE_LAM);
> +
> +    if (!cpu_feature_enabled(X86_FEATURE_FRED))
> +        setup_clear_cpu_cap(X86_FEATURE_FRED);
> +

The following code will work as a generic fix:

	c->x86_capability[i] &= ~DISABLED_MASK(i);

And DISABLED_MASK(x) needs to be defined like DISABLED_MASK_BIT_SET(x).

Thanks!
    Xin