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 09:51:51 +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 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.

Thanks,
Quentin