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 10:08:47 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Nadav Amit <namit@...are.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Willy Tarreau <w@....eu>, "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v2 5/6] x86: Use global pages when PTI is disabled

On 02/15/2018 09:47 AM, Nadav Amit wrote:
> Dave Hansen <dave.hansen@...ux.intel.com> wrote:
>>> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
>>> index c67ef3fb4f35..979c7ec6baab 100644
>>> --- a/arch/x86/mm/tlb.c
>>> +++ b/arch/x86/mm/tlb.c
>>> @@ -74,7 +74,8 @@ static void choose_new_asid(struct mm_struct *next, u64 next_tlb_gen,
>>> 		return;
>>> 	}
>>>
>>> -	if (this_cpu_read(cpu_tlbstate.invalidate_other))
>>> +	if (this_cpu_read(cpu_tlbstate.invalidate_other) &&
>>> +	    !mm_pti_disable(next))
>>> 		clear_asid_other();
>>
>> This isn't obviously correct.  Don't we still need to invalidate other
>> user asids?
> 
> I forgot to regard this question: When you reenable PTI (after switching back
> to 64-bit process), you flush the global pages, so no kernel mappings for the
> 32-bit process are left.

Can you please write up a proper description for this?  It's horribly
complicated, intertwined with global pages, and sets up a dependency
that *ALL* TLB entries invalidated via __flush_tlb_one_kernel() must be
_PAGE_GLOBAL.

How about you actually clear cpu_tlbstate.invalidate_other when you do
the CR4.PGE switching?  That seems a much more direct way and is much
more self-documenting.

That brings up another point: these patches rather ignore cpu_tlbstate.
That leads to confusing code (this) and the double-flushing on context
switch I brought up earlier.  Was this intentional, or is it something
you can reconsider going forward?