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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Fri, 24 Aug 2018 12:31:57 -0700
Message-ID: <CA+55aFzerzTPm94jugheVmWg8dJre94yu+GyZGT9NNZanNx_qw@mail.gmail.com>
Subject: Re: TLB flushes on fixmap changes
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To: Nadav Amit <nadav.amit@gmail.com>
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>

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)?

Maybe worth adding some debugging to verify that?

                Linus