linux-kernel - Re: [PATCH v4 5/5] RISC-V: Add crash kernel support

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Message-ID: <CAMj1kXFfG5Sr+ix9xG3OBM8kgg8ARKNt0HFkL6MafB6vp2_xxQ@mail.gmail.com>
Date:   Wed, 16 Jun 2021 18:30:58 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     Nick Kossifidis <mick@....forth.gr>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>
Subject: Re: [PATCH v4 5/5] RISC-V: Add crash kernel support

On Wed, 16 Jun 2021 at 16:55, Rob Herring <robh+dt@...nel.org> wrote:
>
> +Ard
>
> On Tue, Jun 15, 2021 at 5:29 PM Nick Kossifidis <mick@....forth.gr> wrote:
> >
> > Στις 2021-06-15 22:21, Rob Herring έγραψε:
> > > On Tue, Jun 15, 2021 at 12:48 PM Geert Uytterhoeven
> > > <geert@...ux-m68k.org> wrote:
> > >>
> > >> Hi Nick,
> > >>
> > >> On Tue, Jun 15, 2021 at 8:29 PM Nick Kossifidis <mick@....forth.gr>
> > >> wrote:
> > >> > Στις 2021-06-15 16:19, Geert Uytterhoeven έγραψε:
> > >> > > This does not match
> > >> > > https://github.com/devicetree-org/dt-schema/blob/master/schemas/chosen.yaml#L77:
> > >> > >
> > >> > >     $ref: types.yaml#/definitions/uint64-array
> > >> > >     maxItems: 2
> > >> > >     description:
> > >> > >       This property (currently used only on arm64) holds the memory
> > >> > > range,
> > >> > >       the address and the size, of the elf core header which mainly
> > >> > > describes
> > >> > >       the panicked kernel\'s memory layout as PT_LOAD segments of elf
> > >> > > format.
> > >> > >
> > >> > > Hence "linux,elfcorehdr" should be a property of the /chosen node,
> > >> > > instead of a memory node with a compatible value of "linux,elfcorehdr".
> > >> > >
> > >> >
> > >> > That's a binding for a property on the /chosen node, that as the text
> > >> > says it's defined for arm64 only and the code that handled it was also
> > >>
> > >> That doesn't mean it must not be used on other architectures ;-)
> > >> Arm64 was just the first one to use it...
> > >
> > > It is used on arm64 because memory is often passed by UEFI tables and
> > > not with /memory node. As riscv is also supporting EFI, I'd think they
> > > would do the same.
> > >
> >
> > We've had this discussion before, riscv uses /memory for now and even if
> > we switched to getting memory from ACPI/UEFI tables, the elf core header
> > is passed from the crashed kernel to the kdump kernel, it has nothing to
> > do with UEFI since the bootloader is the kernel itself. Am I missing
> > something ?
>
> I believe if we originally booted using UEFI tables, then those are
> passed the kdump kernel as well. The original DT may have had a
> /memory node, but it's possible it didn't match what was in the UEFI
> tables. So using the DT /memory nodes for kdump could give surprising
> results. I think reserved regions also come from UEFI. Ard can
> probably comment better.
>

Anything that executes in the context of the UEFI boot firmware
(loaders, drivers, etc) may use the UEFI memory allocation routines to
allocate memory, and these allocations are communicated via the UEFI
memory map, not via the /memory node.

So it depends whether it matters if the kexec kernel tramples over
those regions. For kdump scenarios, it might be reasonable, but in the
general case, we should really respect what UEFI tells us about the
memory map when booting via UEFI.