Re: [PATCH 8/8] x86/spec-ctrl: Mitigate the Zen1 DIV leakge

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 14/09/2023 2:12 pm, Jason Andryuk wrote:
> On Wed, Sep 13, 2023 at 6:09 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
> wrote:
>> @@ -955,6 +960,40 @@ static void __init srso_calculations(bool 
>> hw_smt_enabled)
>>          setup_force_cpu_cap(X86_FEATURE_SRSO_NO);
>>  }
>>
>> +/*
>> + * Div leakage is specific to the AMD Zen1 microarchitecure.  Use STIBP as a
>> + * heuristic to select between Zen1 and Zen2 uarches.
>> + */
>> +static bool __init has_div_vuln(void)
>> +{
>> +    if ( !(boot_cpu_data.x86_vendor &
>> +           (X86_VENDOR_AMD | X86_VENDOR_HYGON)) )
>> +        return false;
>> +
>> +    if ( (boot_cpu_data.x86 != 0x17 && boot_cpu_data.x86 != 0x18) ||
>> +         !boot_cpu_has(X86_FEATURE_AMD_STIBP) )
>> +        return false;
>> +
>> +    return true;
>> +}
>> +
>> +static void __init div_calculations(bool hw_smt_enabled)
>> +{
>> +    bool cpu_bug_div = has_div_vuln();
>> +
> Would it make sense to add
>     if ( !cpu_bug_div )
>         return
> ...
>
>> +    if ( opt_div_scrub == -1 )
>> +        opt_div_scrub = cpu_bug_div;
>> +
>> +    if ( opt_div_scrub )
>> +        setup_force_cpu_cap(X86_FEATURE_SC_DIV);
> ...so that div-scrub=1 isn't setting X86_FEATURE_SC_DIV on un-affected
> hardware?  Or do you want to leave command line control in place in
> case it might be needed as a future workaround on other hardware?

All options (where possible) allow for paths to be explicitly activated
on un-affected hardware so we can test this giant mess.

The only cases where we ignore a user choice is when the result will
crash from e.g. #GP due to insufficient microcode.

~Andrew