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

From: Baoquan He <[email-masked]>
Date: Wed, 22 Apr 2020 17:17:18 +0800

On 04/21/20 at 03:29pm, David Hildenbrand wrote:
> >> ACPI SRAT is embeded into efi, need read out the rsdp pointer. If we don't
> >> pass the efi, it won't get the SRAT table correctly, if I remember
> >> correctly. Yeah, I remeber kvm guest can get memory hotplugged with
> >> ACPI only, this won't happen on bare metal though. Need check carefully. 
> >> I have been using kvm guest with uefi firmwire recently.
> > 
> > Yeah, I can imagine that bare metal is different. kvm only uses ACPI.
> > 
> > I'm also asking because of virtio-mem. Memory added via virtio-mem is
> > not part of any efi tables or whatsoever. So I assume the kexec kernel
> > will not detect it automatically (good!), instead load the virtio-mem
> > driver and let it add memory back to the system.
> > 
> > I should probably play with kexec and virtio-mem once I have some spare
> > cycles ... to find out what's broken and needs to be addressed :)
> 
> FWIW, I just gave virtio-mem and kexec/kdump a try.
> 
> a) kdump seems to work. Memory added by virtio-mem is getting dumped.
> The kexec kernel only uses memory in the crash region. The virtio-mem
> driver properly bails out due to is_kdump_kernel().

Right, kdump is not impacted later added memory.

> 
> b) "kexec -s -l" seems to work fine. For now, the kernel does not seem
> to get placed on virtio-mem memory (pure luck due to the left-to-right
> search). Memory added by virtio-mem is not getting added to the e820
> map. Once the virtio-mem driver comes back up in the kexec kernel, the
> right memory is readded.

kexec_file_load just behaves as you tested. It doesn't collect later
added memory to e820 because it uses e820_table_kexec directly to pass
e820 to kexec-ed kernel. However, this e820_table_kexec is only updated
during boot stage. I tried hot adding DIMM after boot, kexec-ed kernel
doesn't have it in e820 during bootup, but it's recoginized and added
when ACPI scanning. I think we should update e820_table_kexec when hot
add/remove memory, at least for DIMM. Not sure if DLPAR, virtio-mem,
balloon will need be added into e820_table_kexec too, and if this is
expected behaviour.

But whatever we do, it won't impact the kexec file_loading, because of
the searching strategy bottom up. Just adding them into e820_table_kexec
will make it consistent with cold reboot which get recognizes and get
them into e820 during bootup.
> 
> c) "kexec -c -l" does not work properly. All memory added by virtio-mem
> is added to the e820 map, which is wrong. Memory that should not be
> touched will be touched by the kexec kernel. I assume kexec-tools just
> goes ahead and adds anything it can find in /proc/iomem (or
> /sys/firmware/memmap/) to the e820 map of the new kernel.
> 
> Due to c), I assume all hotplugged memory (e.g., ACPI DIMMs) is
> similarly added to the e820 map and, therefore, won't be able to be
> onlined MOVABLE easily.

Yes, kexec_load will read memory regions from /sys/firmware/memmap/ or
/proc/iomem. Making it right seems a little harder, we can export them
to /proc/iomem or /sys/firmware/memmap/ with mark them with 'hotplug',
but the attribute that which zone they belongs to is not easy to tell.

We are proactive on widely testing kexec_file_load on x86_64, s390,
arm64 by adding test cases into CKI.

> 
> 
> At least for virtio-mem, I would either have to
> a) Not support "kexec -c -l". A viable option if we would be planning on
> not supporting it either way in the long term. I could block this
> in-kernel somehow eventually.
> 
> b) Teach kexec-tools to leave virtio-mem added memory alone. E.g., by
> indicating it in /proc/iomem in a special way ("System RAM
> (hotplugged)"/"System RAM (virtio-mem)").
> 
> Baoquan, any opinion on that?
> 
> -- 
> Thanks,
> 
> David / dhildenb

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