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

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Date:   Thu, 15 Feb 2018 12:32:50 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>, Nadav Amit <namit@...are.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Willy Tarreau <w@....eu>, Nadav Amit <nadav.amit@...il.com>,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v2 5/6] x86: Use global pages when PTI is disabled

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.