Subject: Re: [PATCH 1/3] kexec: Prevent removal of memory in use by a loaded kexec image

From: Baoquan He <[email-masked]>
Date: Sun, 12 Apr 2020 13:35:07 +0800

On 04/11/20 at 10:30am, Russell King - ARM Linux admin wrote:
> On Sat, Apr 11, 2020 at 11:44:14AM +0800, Baoquan He wrote:
> > Because We tend to use kexec_file_load more and improve/enhance it in the
> > future, and gradually obsolete the old kexec_load interface which this
> > patchset is trying to fix on. 
> 
> That's not going to happen; 32-bit ARM kexec uses the kexec_load
> interface rather than the kexec_file_load version, and I see no one
> with any interest in changing that - and there's users of the former.
> 
> I don't see how it's possible to convert 32-bit ARM kexec to the
> kexec_file_load interface - this assumes that all you have are the
> kernel, initrd, and commandline, but on 32-bit ARM kexec, we have
> kernel, initrd and the dtb blob which the user can specify.

Well, I understand what you said about 32-bit ARM support with only
kexec_old support thing. That's why I said we tend to obsolete it
'GRADUALLY'. It's the existing users who are using kexec_load, and the
ARCHes which only has kexec_load, make us have to transfer to
kexec_file_load gradually.

Comparing with kexec_load, kexec_file_load has only one disadvantage,
that is some ARCHes only have kexec_load. Otherwise, kexec_file_load
benefits kexec/kdump developping/maintaining very much. The loading job
of kexec_file_load is mostly done in kernel, we can get whatever we
want about kernel information very conveniently to do anything needed.
For the kexec_load interface, the loading job is mostly done in
userspace, we have to export kernel information to procfs, sysfs, etc,
then parse them in kexec_tools, finally passed it to kernel part of
kexec loading.

The gradual obsoleting means we may only add
feature/improvement/enhancement to kexec_file_load. And if a bug fix is
needed for both kexec_load and kexec_file_load, and the fix is very
complicated, we may only fix it in kexec_file_load too. Kexec_file_load
interface is suggested to add if does't have, just port user space part
to kernel as x86/s390/arm64 have done.

Surely, it doesn't mean we don't fix the critical/blocker bug with
kexec_load loading. We still try to do, just are not so eager. In the
existing product environment, the kexec_load is used, just keep using
it. Do we bother to change it to kexec_file_load, e.g in our RHEL7
distros? Certainly not. But in our new product, we will change to use
kexec_file_load interface. I guess this is similar with arm64. The
advantage and benefit have been told in the 2nd paragraph.

As for 32-bit ARM, is it like the old product, we have many in-use systems
deployed in customers' laboratory? Wondering if ARM continues designing
new 32-bit ARM cpu, and some companies continue producing tons of 32-bit ARM
cpus. If yes, I think we need continue taking care of kexec_load if
32-bit ARM can't convert to kexec_file_load. If not, it may be not a
barrier when we consider converting kexec_load to kexec_file_load in
other ARCHes. We just need keep using it, try to fix those critical/blocker
bug in kexec_load interface if encountered.

Finally, comning back to this patchset itself, the issue James spotted
is not so ciritical, I would say. When I do kexec jumping, I will do
loading firstly, then trigge jumping. I can think of the case that
people may load kexec-ed kernel, then do something else, later she/he
triggers the kexec jumping. These are not necessary steps. As Dave and I
replied to James in the cover-letter thread, adding a systemd service of
kexec loading, monitor hotplug uevent, reload it if any hot remove
happened. This is quite easy to do, I don't see any problem with it, and
why we don't do like this. 

My personal opinion, please tell if I miss anything.

> 
> So, if we wanted to obsolete the kexec_load interface, _first_ there
> needs to be a way to provide users with the existing functionality
> they have already in place on 32-bit ARM - otherwise we're looking
> at a userspace regression.  Especially as kexec_file_load takes
> precedence on some distro patched versions of the kexec tool,
> irrespective of which interface the user requests of the tool.
> 
> -- 
> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
> FTTC broadband for 0.8mile line in suburbia: sync at 10.2Mbps down 587kbps up
> 

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