linux-kernel - Re: [PATCH v21 5/7] x86/crash: add x86 crash hotplug support

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Date:   Thu, 27 Apr 2023 16:49:56 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Hari Bathini <hbathini@...ux.ibm.com>
Cc:     Eric DeVolder <eric.devolder@...cle.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, vgoyal@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, nramas@...ux.microsoft.com, thomas.lendacky@....com,
        robh@...nel.org, efault@....de, rppt@...nel.org, david@...hat.com,
        sourabhjain@...ux.ibm.com, konrad.wilk@...cle.com,
        boris.ostrovsky@...cle.com
Subject: Re: [PATCH v21 5/7] x86/crash: add x86 crash hotplug support

On 04/27/23 at 12:39pm, Hari Bathini wrote:
> Hi Eric,
> 
> On 04/04/23 11:33 pm, Eric DeVolder wrote:
> > When CPU or memory is hot un/plugged, or off/onlined, the crash
> > elfcorehdr, which describes the CPUs and memory in the system,
> > must also be updated.
> > 
> > The segment containing the elfcorehdr is identified at run-time
> > in crash_core:crash_handle_hotplug_event(), which works for both
> > the kexec_load() and kexec_file_load() syscalls. A new elfcorehdr
> > is generated from the available CPUs and memory into a buffer,
> > and then installed over the top of the existing elfcorehdr.
> > 
> > In the patch 'kexec: exclude elfcorehdr from the segment digest'
> > the need to update purgatory due to the change in elfcorehdr was
> > eliminated.  As a result, no changes to purgatory or boot_params
> > (as the elfcorehdr= kernel command line parameter pointer
> > remains unchanged and correct) are needed, just elfcorehdr.
> > 
> > To accommodate a growing number of resources via hotplug, the
> > elfcorehdr segment must be sufficiently large enough to accommodate
> > changes, see the CRASH_MAX_MEMORY_RANGES description. This is used
> > only on the kexec_file_load() syscall; for kexec_load() userspace
> > will need to size the segment similarly.
> > 
> > To accommodate kexec_load() syscall in the absence of
> 
> Firstly, thanks! This series is a nice improvement to kdump support
> in hotplug environment.
> 
> One concern though is that this change assumes corresponding support
> in kexec-tools. Without that support kexec_load would fail to boot
> with digest verification failure, iiuc.

Eric has posted patchset to modify kexec_tools to support that, please
see the link Eric pasted in the cover letter.

http://lists.infradead.org/pipermail/kexec/2022-October/026032.html