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

[David Hildenbrand]

>> 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().

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.

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.


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