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

["H. Peter Anvin" <hpa@zytor.com>]

On July 22, 2025 2:08:28 AM PDT, Xin Li <xin@zytor.com> wrote:
>
>> On Jul 22, 2025, at 12:54 AM, Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com> 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@intel.com>
>
>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

The easiest thing would be to initialize the setup disabled mask with the DISABLED bitmask instead of zero. This can be done statically; if it isn't already the awk script can produce the disabled bitmask in array form.