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

[Hari Bathini <hbathini@linux.ibm.com>]

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.

I would suggest a flag to advertise to the kernel that kexec-tools/
userspace wants in-kernel update. Something like KEXEC_IN_KERNEL_UPDATE
on top of existing flags like KEXEC_ON_CRASH & KEXEC_PRESERVE_CONTEXT.
This flag can be used to decide whether in-kernel update needs to be
enforced or not. That should make transition to this change smoother
without having to break userspace.

Thanks
Hari