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

[David Hildenbrand]

On 22.04.20 11:17, Baoquan He wrote:
> 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.

Yeah, I think whatever a cold-booted kernel will see is what kexec-ed
kernel should see. Not more, not less.

Regarding virtio-mem: Not in e820 on cold-boot.
Regarding DIMMs: DIMMs under KVM will never show up in the e820 map
IIRC. I think on real HW it can be different.

-- 
Thanks,

David / dhildenb