On Thu 19-07-18 23:17:53, Baoquan He wrote:

Kexec has been a formal feature in our distro, and customers owning
those kind of very large machine can make use of this feature to speed
up the reboot process. On uefi machine, the kexec_file loading will
search place to put kernel under 4G from top to down. As we know, the
1st 4G space is DMA32 ZONE, dma, pci mmcfg, bios etc all try to consume
it. It may have possibility to not be able to find a usable space for
kernel/initrd. From the top down of the whole memory space, we don't
have this worry. 

I do not have the full context here but let me note that you should be
careful when doing top-down reservation because you can easily get into
hotplugable memory and break the hotremove usecase. We even warn when
this is done. See memblock_find_in_range_node
-- 
Michal Hocko
SUSE Labs