linux-kernel - Re: [PATCH v12 7/7] x86/crash: Add x86 crash hotplug support

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Message-ID: <YzcqE1RVtPcuLlxN@zn.tnic>
Date:   Fri, 30 Sep 2022 19:40:35 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Eric DeVolder <eric.devolder@...cle.com>
Cc:     Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>, david@...hat.com,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, bhe@...hat.com, vgoyal@...hat.com,
        tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
        hpa@...or.com, nramas@...ux.microsoft.com, thomas.lendacky@....com,
        robh@...nel.org, efault@....de, rppt@...nel.org,
        sourabhjain@...ux.ibm.com, linux-mm@...ck.org
Subject: Re: [PATCH v12 7/7] x86/crash: Add x86 crash hotplug support

On Fri, Sep 30, 2022 at 12:11:26PM -0500, Eric DeVolder wrote:
> There is of course a way to enumerate the memory regions in use on the
> machine, that is not what this code needs. In order to compute the maximum
> buffer size needed (this buffer size is computed once), the count of the
> maximum number of memory regions possible (even if not currently in use) is
> what is needed.

Isn't that max number documented somewhere in memory hotplug docs?

Because then you don't need that Kconfig item either. Imagine you're a
distro kernel distributor and you want crash to work on all machines
your kernel works.

So you go and set that number to max. And that would be the 99% of the
kernel configs out there.

Which means, you can just set it to max without a Kconfig item.

> Oh, that would be an error of haste on my part. This should be:
>   depends on CRASH_DUMP && MEMORY_HOTPLUG

You need a Kconfig item which enables all this gunk as MEMORY_HOTPLUG is
not a omnipresent feature. And that Kconfig item should depend on the
other Kconfig items of the technology you need.

> Baoquan pointed me to:
> 
> https://lore.kernel.org/lkml/cover.1656659357.git.naveen.n.rao@linux.vnet.ibm.com/T/

In that thread says:

"- arch_kexec_apply_relocations_add() is only overridden by x86 and s390.
  Retain the function prototype for those and move the weak
  implementation into the header as a static inline for other
  architectures."

So yes, that's even better.

-- 
Regards/Gruss,
    Boris.

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