linux-kernel - Re: TLB flushes on fixmap changes

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Message-Id: <9A38D3F4-2F75-401D-8B4D-83A844C9061B@gmail.com>
Date:   Fri, 24 Aug 2018 13:24:54 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Benjamin Herrenschmidt <benh@....ibm.com>,
        Nick Piggin <npiggin@...il.com>,
        Andrew Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Rik van Riel <riel@...riel.com>,
        Jann Horn <jannh@...gle.com>,
        Adin Scannell <ascannell@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        David Miller <davem@...emloft.net>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: TLB flushes on fixmap changes

at 12:31 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Fri, Aug 24, 2018 at 11:36 AM Nadav Amit <nadav.amit@...il.com> wrote:
>>> Urgh.. weren't the fixmaps per cpu? Bah, I remember looking at this
>>> during PTI, but I seem to have forgotten everything again.
>> 
>> [ Changed the title. Sorry for hijacking the thread. ]
>> 
>> Since:
>> 
>> native_set_fixmap()->set_pte_vaddr()->pgd_offset_k()
> 
> The fixmaps should be entirely fixed after bootup to constant
> mappings, except for the KMAP ones, and they are indexed per-cpu.
> 
> That's what my mental model is, at least.
> 
> Can you actually find something that changes the fixmaps after boot
> (again, ignoring kmap)?

At least the alternatives mechanism appears to do so.
 
IIUC the following path is possible when adding a module:

	jump_label_add_module()
	->__jump_label_update()
	->arch_jump_label_transform()
	->__jump_label_transform()
	->text_poke_bp()
	->text_poke()
	->set_fixmap()
	
And a similar path can happen when static_key_enable/disable() is called.