linux-kernel - Re: [PATCH 04/23] x86, tlb: make CR4-based TLB flushes more robust

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Message-ID: <CALCETrVX6-wk08StxPSafJH5q7awXXwbE9Pz_Axf+17cH7BOdA@mail.gmail.com>
Date:   Wed, 1 Nov 2017 04:18:38 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        moritz.lipp@...k.tugraz.at,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        michael.schwarz@...k.tugraz.at,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 04/23] x86, tlb: make CR4-based TLB flushes more robust

On Wed, Nov 1, 2017 at 3:56 AM, Kirill A. Shutemov <kirill@...temov.name> wrote:
> On Wed, Nov 01, 2017 at 03:38:23AM -0700, Andy Lutomirski wrote:
>> On Wed, Nov 1, 2017 at 3:11 AM, Kirill A. Shutemov <kirill@...temov.name> wrote:
>> > On Wed, Nov 01, 2017 at 01:01:45AM -0700, Andy Lutomirski wrote:
>> >> On Tue, Oct 31, 2017 at 3:31 PM, Dave Hansen
>> >> <dave.hansen@...ux.intel.com> wrote:
>> >> >
>> >> > Our CR4-based TLB flush currently requries global pages to be
>> >> > supported *and* enabled.  But, we really only need for them to be
>> >> > supported.  Make the code more robust by alllowing X86_CR4_PGE to
>> >> > clear as well as set.
>> >> >
>> >> > This change was suggested by Kirill Shutemov.
>> >>
>> >> I may have missed something, but why would be ever have CR4.PGE off?
>> >
>> > This came out from me thinking on if we can disable global pages by not
>> > turning on CR4.PGE instead of making _PAGE_GLOBAL zero.
>> >
>> > Dave decided to not take this path, but this change would make
>> > __native_flush_tlb_global_irq_disabled() a bit less fragile in case
>> > if the situation would change in the future.
>>
>> How about just adding a VM_WARN_ON_ONCE, then?
>
> What's wrong with xor? The function will continue to work this way even if
> CR4.PGE is disabled.

That's true.  OTOH, since no one is actually proposing doing that,
there's an argument that people should get warned and therefore be
forced to think about it.