linux-kernel - Re: [PATCH v5 07/16] kexec: add Kexec HandOver (KHO) generation helpers

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Message-ID: <20250321133447.GA251739@nvidia.com>
Date: Fri, 21 Mar 2025 10:34:47 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Changyuan Lyu <changyuanl@...gle.com>
Cc: linux-kernel@...r.kernel.org, graf@...zon.com,
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Subject: Re: [PATCH v5 07/16] kexec: add Kexec HandOver (KHO) generation
 helpers

On Wed, Mar 19, 2025 at 06:55:42PM -0700, Changyuan Lyu wrote:
> From: Alexander Graf <graf@...zon.com>
> 
> Add the core infrastructure to generate Kexec HandOver metadata. Kexec
> HandOver is a mechanism that allows Linux to preserve state - arbitrary
> properties as well as memory locations - across kexec.
> 
> It does so using 2 concepts:
> 
>   1) State Tree - Every KHO kexec carries a state tree that describes the
>      state of the system. The state tree is represented as hash-tables.
>      Device drivers can add/remove their data into/from the state tree at
>      system runtime. On kexec, the tree is converted to FDT (flattened
>      device tree).

Why are we changing this? I much prefered the idea of having recursive
FDTs than this notion copying eveything into tables then out into FDT?
Now that we have the preserved pages mechanism there is a pretty
direct path to doing recursive FDT.

I feel like this patch is premature, it should come later in the
project along with a stronger justification for this approach.

IHMO keep things simple for this series, just the very basics.

> +int register_kho_notifier(struct notifier_block *nb)
> +{
> +	return blocking_notifier_chain_register(&kho_out.chain_head, nb);
> +}
> +EXPORT_SYMBOL_GPL(register_kho_notifier);

And another different set of notifiers? :(

> +static int kho_finalize(void)
> +{
> +	int err = 0;
> +	void *fdt;
> +
> +	fdt = kvmalloc(kho_out.fdt_max, GFP_KERNEL);
> +	if (!fdt)
> +		return -ENOMEM;

We go to all the trouble of keeping track of stuff in dynamic hashes
but still can't automatically size the fdt and keep the dumb uapi to
have the user say? :( :(

Jason