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

[Baoquan He <bhe@redhat.com>]

On 04/27/23 at 10:26pm, Hari Bathini wrote:
> On 27/04/23 2:19 pm, Baoquan He wrote:
> > 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
> 
> Right, Baoquan.
> 
> I did see that and if I read the code correctly, without that patchset
> kexec_load would fail. Not with an explicit error that hotplug support
> is missing or such but it would simply fail to boot into capture kernel
> with digest verification failure.
> 
> My suggestion was to avoid that userspace tool breakage for older
> kexec-tools version by introducing a new kexec flag that can tell
> kernel that kexec-tools is ready to use this in-kernel update support.
> So, if kexec_load happens without the flag, avoid doing an in-kernel
> update on hotplug. I hope that clears the confusion.

Yeah, sounds like a good idea. It may be extended in later patch.