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Subject: Re: TLB flushes on fixmap changes
From: Nadav Amit <nadav.amit@gmail.com>
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Date: Fri, 24 Aug 2018 13:24:54 -0700
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>, Will Deacon <will.deacon@arm.com>, Benjamin Herrenschmidt <benh@au1.ibm.com>, Nick Piggin <npiggin@gmail.com>, Andrew Lutomirski <luto@kernel.org>, the arch/x86 maintainers <x86@kernel.org>, Borislav Petkov <bp@alien8.de>, Rik van Riel <riel@surriel.com>, Jann Horn <jannh@google.com>, Adin Scannell <ascannell@google.com>, Dave Hansen <dave.hansen@intel.com>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, David Miller <davem@davemloft.net>, Martin Schwidefsky <schwidefsky@de.ibm.com>, Michael Ellerman <mpe@ellerman.id.au>

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

> On Fri, Aug 24, 2018 at 11:36 AM Nadav Amit <nadav.amit@gmail.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.