[PATCH v3 1/3] kexec_load: Use new kexec flag for hotplug support

[Baoquan He]

On 07/08/24 at 01:25pm, Sourabh Jain wrote:
> Hello Baoquan,
> 
> On 08/07/24 07:09, Baoquan He wrote:
> > Hi Sourabh,
> > 
> > On 07/07/24 at 08:54pm, Sourabh Jain wrote:
> > > Kernel commit 79365026f869 (crash: add a new kexec flag for hotplug
> > > support) has introduced a new kexec flag to generalize hotplug support.
> > > The newly introduced kexec flags for hotplug allow architectures to
> > > exclude all the required kexec segments from SHA calculation so that
> > > the kernel can update them on hotplug events. This was not possible
> > > earlier with the KEXEC_UPDATE_ELFCOREHDR kexec flags since it was added
> > > only for the elfcorehdr segment.
> > > 
> > > To enable architectures to control the list of kexec segments to exclude
> > > when hotplug support is enabled, add a new architecture-specific
> > > function named arch_do_exclude_segment. During the SHA calculation, this
> > > function gets called to let the architecture decide whether a specific
> > > kexec segment should be considered for SHA calculation or not.
> > > 
> > > Note: To avoid breaking backward compatibility, the new kexec flag
> > > KEXEC_CRASH_HOTPLUG_SUPPORT is not used for x86 for now.
> > For x86, both KEXEC_UPDATE_ELFCOREHDR and KEXEC_CRASH_HOTPLUG_SUPPORT
> > should be OK for kexec_file_load.
> 
> Do we even need these flags for kexec_file_load at all?
> My understanding is that these flags are only needed for the kexec_load
> system call.

Oh, sorry, my bad, I must have mixed this with KEXEC_FILE_DEBUG I
earlier added when I checked this patchset. I think everything is
like what you said.
> 
> 
> > Your change will make a difference
> > between kexec_load and kexec_file_load.
> 
> I am confused by the above statement.
> 
> Given that we don't even send any of the above flags for kexec_file_load, I
> am not
> sure how these changes make a difference between the two system calls.
> 
> > But I agree with you on the
> > backward cmpatibility with KEXEC_CRASH_HOTPLUG_SUPPORT flag.
> > 
> > Anyway, if it's in a hurry to catch up with Simon's new release, this is
> > fine, we can change it later.
> 
> It would be great if we could consider this patch series for the next
> release, but not at
> the cost of breaking any backward compatibility for x86. If you think these
> changes are
> breaking anything for any kernel version, I would prefer to update my patch
> series.
> 
> > Otherwise, we may be better to remove the
> > difference, namely, not making x86 only be able to accept
> > KEXEC_UPDATE_ELFCOREHDR flag on kexec_load. My personal opinion
> 
> On x86, passing the KEXEC_CRASH_HOTPLUG_SUPPORT kexec bit to kernel versions
> 6.5 to 6.9
> with the kexec_load system call will fail with -EINVAL. However, from kernel
> 6.10 onward,
> both KEXEC_UPDATE_ELFCOREHDR and KEXEC_CRASH_HOTPLUG_SUPPORT kexec bits are
> acceptable for x86.
> 
> My proposal is to use KEXEC_UPDATE_ELFCOREHDR on x86 for some time (maybe a
> couple of kernel releases),
> and eventually switch to KEXEC_CRASH_HOTPLUG_SUPPORT for x86 as well.
> 
> This proposal of shifting to the KEXEC_CRASH_HOTPLUG_SUPPORT kexec bit for
> x86 is also mentioned in the
> comment for the get_hotplug_kexec_flag function.
> 
> Please let me know your opinion.

It sounds like a good plan, thanks for the effort.