Re: [PATCH v18 02/17] x86/setup: Move xen_pv_domain() check and insert_resource() to setup_arch()

[Borislav Petkov <bp@alien8.de>]

On Wed, Dec 22, 2021 at 09:08:05PM +0800, Zhen Lei wrote:
> From: Chen Zhou <chenzhou10@huawei.com>
> 
> We will make the functions reserve_crashkernel() as generic, the
> xen_pv_domain() check in reserve_crashkernel() is relevant only to
> x86,

Why is that so? Is Xen-PV x86-only?

> the same as insert_resource() in reserve_crashkernel[_low]().

Why?

Looking at

  0212f9159694 ("x86: Add Crash kernel low reservation")

it *surprisingly* explains why that resources thing is being added:

    We need to add another range in /proc/iomem like "Crash kernel low",
    so kexec-tools could find that info and append to kdump kernel
    command line.

Then,

  157752d84f5d ("kexec: use Crash kernel for Crash kernel low")

renamed it because, as it states, kexec-tools was taught to handle
multiple resources of the same name.

So why does kexec-tools on arm *not* need those iomem resources? How
does it parse the ranges there? Questions over questions...

So last time I told you to sit down and take your time with this cleanup.
From reading this here, it doesn't look like it. Rather, it looks like
hastily done in a hurry and hurrying stuff doesn't help you one bit - it
actually makes it worse.

Your commit messages need to explain *why* a change is being done and
why is that ok. This one doesn't.

> @@ -1120,7 +1109,17 @@ void __init setup_arch(char **cmdline_p)
>  	 * Reserve memory for crash kernel after SRAT is parsed so that it
>  	 * won't consume hotpluggable memory.
>  	 */
> -	reserve_crashkernel();
> +#ifdef CONFIG_KEXEC_CORE
> +	if (xen_pv_domain())
> +		pr_info("Ignoring crashkernel for a Xen PV domain\n");

This is wrong - the check is currently being done inside
reserve_crashkernel(), *after* it has parsed a crashkernel= cmdline
correctly - and not before.

Your change would print on Xen PV, regardless of whether it has received
crashkernel= on the cmdline or not.

This is exactly why I say that making those functions generic and shared
might not be such a good idea, after all, because then you'd have to
sprinkle around arch-specific stuff.

One of the ways how to address this particular case here would be:

1. Add a x86-specific wrapper around parse_crashkernel() which does
all the parsing. When that wrapper finishes, you should have parsed
everything that has crashkernel= on the cmdline.

2. At the end of that wrapper, you do arch-specific checks and setup
like the xen_pv_domain() one.

3. Now, you do reserve_crashkernel(), if those checks pass.

The question is, whether the flow on arm64 can do the same. Probably but
it needs careful auditing.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette