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Subject: Re: [PATCH RFC v2 5/6] x86: Use global pages when PTI is disabled
To:     Nadav Amit <namit@vmware.com>
References: <20180215163602.61162-1-namit@vmware.com>
 <20180215163602.61162-6-namit@vmware.com>
 <10c21933-fe93-ccad-b315-2a7ca1e917a4@linux.intel.com>
 <E2528628-52A6-444F-BE02-0C69D5E52726@vmware.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Willy Tarreau <w@1wt.eu>, "x86@kernel.org" <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
From:   Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <7b48990b-0a56-a26d-5d98-fe305331caec@linux.intel.com>
Date:   Thu, 15 Feb 2018 10:08:47 -0800
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On 02/15/2018 09:47 AM, Nadav Amit wrote:
> Dave Hansen <dave.hansen@linux.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?