linux-kernel - Re: [RFT PATCH v3 1/9] RISC-V: Move DT mapping outof fixmap

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Date:   Tue, 21 Jul 2020 12:02:39 +0300
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Atish Patra <atishp@...shpatra.org>,
        Atish Patra <atish.patra@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Anup Patel <anup.patel@....com>,
        Greentime Hu <greentime.hu@...ive.com>,
        Kees Cook <keescook@...omium.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Mark Rutland <mark.rutland@....com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Will Deacon <will@...nel.org>, Zong Li <zong.li@...ive.com>,
        Heinrich Schuchardt <xypron.glpk@....de>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFT PATCH v3 1/9] RISC-V: Move DT mapping outof fixmap

On Tue, 21 Jul 2020 at 11:57, Arnd Bergmann <arnd@...db.de> wrote:
>
> On Tue, Jul 21, 2020 at 6:18 AM Atish Patra <atishp@...shpatra.org> wrote:
> > On Sat, Jul 18, 2020 at 2:24 AM Arnd Bergmann <arnd@...db.de> wrote:
> > > On Sat, Jul 18, 2020 at 3:05 AM Atish Patra <atishp@...shpatra.org> wrote:
> > > > That's what the original code was doing. A fixmap entry was added to
> > > > map the original fdt
> > > > location to a virtual so that parse_dtb can be operated on a virtual
> > > > address. But we can't map
> > > > both FDT & early ioremap within a single PMD region( 2MB ). That's why
> > > > we removed the DT
> > > > mapping from the fixmap to .bss section. The other alternate option is
> > > > to increase the fixmap space to 4MB which seems more fragile.
> > >
> > > Could the original location just be part of the regular linear mapping of all
> > > RAM?
> >
> > No. Because we don't map the entire RAM until setup_vm_final().
> > We need to parse DT before setup_vm_final() to get the memblocks and
> > reserved memory regions.
>
> Ok, I see how you create a direct mapping for the kernel image, plus
> the fixmap for the dtb in setup_vm(), and how moving the dtb into the
> kernel image simplifies that.
>
> I'm still wondering why you can't do the same kind of PGD mapping
> for the dtb that you do for the vmlinux, creating linear page table
> entries exactly for the location that holds the dtb, from dtb_pa to
> dtb_pa+((struct fdt_header*)dtb_pa)->totalsize.
>

On arm64, we limit the size of the DT to 2MB, and reserve a pair of
PMD entries adjacent to the fixmap so we can map it r/o statically
using huge pages without using fixmap/early_ioremap slots. (Using a
pair of PMD entries allows the DT to appear at any alignment in
memory, given that PMD entries cover 2 MB each on 4k pages kernels)