linux-kernel - Re: TLB flushes on fixmap changes

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Date:   Fri, 24 Aug 2018 12:31:57 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Nadav Amit <nadav.amit@...il.com>
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

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)?

Maybe worth adding some debugging to verify that?

                Linus