Re: [PATCH RFC v2 5/6] x86: Use global pages when PTI is disabled

[Dave Hansen]

On 02/15/2018 11:53 AM, Andy Lutomirski wrote:
>> --- a/arch/x86/include/asm/tlbflush.h
>> +++ b/arch/x86/include/asm/tlbflush.h
>> @@ -319,6 +319,12 @@ static inline void set_cpu_pti_disable(unsigned short disable)
>>         WARN_ON_ONCE(preemptible());
>>
>>         pti_update_user_cs64(cpu_pti_disable(), disable);
>> +       if (__supported_pte_mask & _PAGE_GLOBAL) {
>> +               if (disable)
>> +                       cr4_set_bits(X86_CR4_PGE);
>> +               else
>> +                       cr4_clear_bits(X86_CR4_PGE);
>> +       }
> This will be *extremely* slow, and I don't see the point at all.  What
> are you accomplishing here?

It won't be slow if you always run compat processes, I guess.

But mixing these in here will eat a big chunk of the benefit of having
global pages (or PCIDs for that matter) in the first place.