Re: [PATCH v4 3/6] ieee1275: support runtime memory claiming

[Robbie Harwood <rharwood@redhat.com>]

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Stefan Berger <stefanb@linux.ibm.com> writes:

> From: Daniel Axtens <dja@axtens.net>
>
> On powerpc-ieee1275, we are running out of memory trying to verify
> anything. This is because:
>
>  - we have to load an entire file into memory to verify it. This is
>    difficult to change with appended signatures.
>  - We only have 32MB of heap.
>  - Distro kernels are now often around 30MB.
>
> So we want to be able to claim more memory from OpenFirmware for our heap
> at runtime.
>
> There are some complications:
>
>  - The grub mm code isn't the only thing that will make claims on
>    memory from OpenFirmware:
>
>     * PFW/SLOF will have claimed some for their own use.
>
>     * The ieee1275 loader will try to find other bits of memory that we
>       haven't claimed to place the kernel and initrd when we go to boot.
>
>     * Once we load Linux, it will also try to claim memory. It claims
>       memory without any reference to /memory/available, it just starts
>       at min(top of RMO, 768MB) and works down. So we need to avoid this
>       area. See arch/powerpc/kernel/prom_init.c as of v5.11.
>
>  - The smallest amount of memory a ppc64 KVM guest can have is 256MB.
>    It doesn't work with distro kernels but can work with custom kernels.
>    We should maintain support for that. (ppc32 can boot with even less,
>    and we shouldn't break that either.)
>
>  - Even if a VM has more memory, the memory OpenFirmware makes available
>    as Real Memory Area can be restricted. Even with our CAS work, an LPAR
>    on a PowerVM box is likely to have only 512MB available to OpenFirmware
>    even if it has many gigabytes of memory allocated.
>
> What should we do?
>
> We don't know in advance how big the kernel and initrd are going to be,
> which makes figuring out how much memory we can take a bit tricky.
>
> To figure out how much memory we should leave unused, I looked at:
>
>  - an Ubuntu 20.04.1 ppc64le pseries KVM guest:
>     vmlinux: ~30MB
>     initrd:  ~50MB
>
>  - a RHEL8.2 ppc64le pseries KVM guest:
>     vmlinux: ~30MB
>     initrd:  ~30MB
>
> So to give us a little wriggle room, I think we want to leave at least
> 128MB for the loader to put vmlinux and initrd in memory and leave Linux
> with space to satisfy its early allocations.
>
> Allow other space to be allocated at runtime.
>
> Tested-by: Stefan Berger <stefanb@linux.ibm.com>
> Signed-off-by: Daniel Axtens <dja@axtens.net>

Reviewed-by: Robbie Harwood <rharwood@redhat.com>

Be well,
--Robbie

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