Re: [PATCH] riscv: reserve DTB before possible memblock allocation

From: Conor Dooley <conor@kernel.org>
To: Alexandre Ghiti <alexghiti@rivosinc.com>
Cc: Woody Zhang <woodylab@foxmail.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] riscv: reserve DTB before possible memblock allocation
Date: Sat, 10 Jun 2023 16:41:17 +0100	[thread overview]
Message-ID: <20230610-ability-mockup-f6aa71bad6a1@spud> (raw)
In-Reply-To: <CAHVXubggydGfNu3OzcxWXREJbB+G9dmr9sFD7vXhVxbG-N58Pg@mail.gmail.com>

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On Thu, Jun 08, 2023 at 09:49:44AM +0200, Alexandre Ghiti wrote:
> On Wed, Jun 7, 2023 at 8:17 PM Conor Dooley <conor@kernel.org> wrote:
> >
> > +CC Alex, you should take a look at this patch.
> >
> > On Wed, Jun 07, 2023 at 09:35:19PM +0800, Woody Zhang wrote:
> > > It's possible that early_init_fdt_scan_reserved_mem() allocates memory
> > > from memblock for dynamic reserved memory in `/reserved-memory` node.
> > > Any fixed reservation must be done before that to avoid potential
> > > conflicts.
> > >
> > > Reserve the DTB in memblock just after early scanning it.
> >
> > The rationale makes sense to me, I am just wondering what compelling
> > reason there is to move it away from the memblock_reserve()s for the
> > initd and vmlinux? Moving it above early_init_fdt_scan_reserved_mem()
> > should be the sufficient minimum & would keep things together.

> Thanks Conor.
> 
> So the patch looks good to me.
> 
> But I find this fragile:
> 
> - we do not check memblock_reserve() return value to make sure the
> reservation really happened (and quickly looking at the code, I'm not
> even sure it returns an error if the region was already allocated).
> - we have to make sure no memblock allocation happens before setup_bootmem().
> - we also have to check that no fixed memblock_reserve() happens after.
> 
> The last 2 points may sound natural, but we'll have to take great care
> when adding some code around here. I'm working on an "early boot
> document" and I'll add something about that, but a runtime thing would
> be way better IMO.
> 
> You can add:
> 
> Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>

btw Alex/Woody, what is the appropriate Fixes tag here?

Cheers,
Conor.

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