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

[David Hildenbrand <david@redhat.com>]

On 21.04.20 15:29, 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().
> 
> 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)").

I just realized, that *not* creating /sys/firmware/memmap/ entries for
virtio-mem memory seems to be the right thing to do.


-- 
Thanks,

David / dhildenb