linux-kernel - Re: [RFC PATCH v2 15/26] of/fdt: Introduce early_init_dt_add_memory

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Date:   Tue, 12 Jan 2021 16:15:28 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Frank Rowand <frowand.list@...il.com>,
        devicetree@...r.kernel.org, android-kvm@...gle.com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        "open list:KERNEL VIRTUAL MACHINE FOR ARM64 (KVM/arm64)" 
        <kvmarm@...ts.cs.columbia.edu>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Fuad Tabba <tabba@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        David Brazdil <dbrazdil@...gle.com>
Subject: Re: [RFC PATCH v2 15/26] of/fdt: Introduce
 early_init_dt_add_memory_hyp()

On Tuesday 12 Jan 2021 at 09:53:36 (-0600), Rob Herring wrote:
> On Tue, Jan 12, 2021 at 8:26 AM Quentin Perret <qperret@...gle.com> wrote:
> >
> > On Tuesday 12 Jan 2021 at 08:10:47 (-0600), Rob Herring wrote:
> > > On Tue, Jan 12, 2021 at 3:51 AM Quentin Perret <qperret@...gle.com> wrote:
> > > >
> > > > On Monday 11 Jan 2021 at 08:45:10 (-0600), Rob Herring wrote:
> > > > > On Fri, Jan 8, 2021 at 6:16 AM Quentin Perret <qperret@...gle.com> wrote:
> > > > > >
> > > > > > Introduce early_init_dt_add_memory_hyp() to allow KVM to conserve a copy
> > > > > > of the memory regions parsed from DT. This will be needed in the context
> > > > > > of the protected nVHE feature of KVM/arm64 where the code running at EL2
> > > > > > will be cleanly separated from the host kernel during boot, and will
> > > > > > need its own representation of memory.
> > > > >
> > > > > What happened to doing this with memblock?
> > > >
> > > > I gave it a go, but as mentioned in v1, I ran into issues for nomap
> > > > regions. I want the hypervisor to know about these memory regions (it's
> > > > possible some of those will be given to protected guests for instance)
> > > > but these seem to be entirely removed from the memblocks when using DT:
> > > >
> > > > https://elixir.bootlin.com/linux/latest/source/drivers/of/fdt.c#L1153
> > > >
> > > > EFI appears to do things differently, though, as it 'just' uses
> > > > memblock_mark_nomap() instead of actively removing the memblock. And that
> > > > means I could actually use the memblock API for EFI, but I'd rather
> > > > have a common solution. I tried to understand why things are done
> > > > differently but couldn't find an answer and kept things simple and
> > > > working for now.
> > > >
> > > > Is there a good reason for not using memblock_mark_nomap() with DT? If
> > > > not, I'm happy to try that.
> > >
> > > There were 2 patches to do that, but it never got resolved. See here[1].
> >
> > Thanks. So the DT stuff predates the introduction of memblock_mark_nomap,
> > that's why...
> >
> > By reading the discussions, [1] still looks a sensible patch on its own,
> > independently from the issue Nicolas tried to solve. Any reason for not
> > applying it?
> 
> As I mentioned in the thread, same patch with 2 different reasons. So
> I just wanted a better commit message covering both.

Sorry if I'm being thick, but I'm not seeing it. How are they the same?
IIUC, as per Nicolas' last reply, using memblock_mark_nomap() does not
solve his issue with a broken DT. These 2 patches address two completely
separate issues no?

Thanks,
Quentin