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

[James Morse <james.morse@arm.com>]

Hi guys,

On 22/04/2020 17:40, David Hildenbrand wrote:
>>>> Usually somewhere in the loaded image
>>>> is a copy of the memory map at the time the kexec kernel was loaded.
>>>> That will invalidate the memory map as well.
>>>
>>> Ah, unconditionally. Sure, x86 needs this.
>>> (arm64 re-discovers the memory map from firmware tables after kexec)

> Does this include hotplugged DIMMs e.g., under KVM?

If you advertise hotplugged memory to the guest using ACPI, yes.

We don't have a practical mechanism to pass 'fact's about the platform between kernels,
instead we rely on those facts being discoverable, or described by firmware.


>>>> All of this should be for a very brief window of a few seconds, as
>>>> the loaded kexec image is quite short.
>>>
>>> It seems I'm the outlier anticipating anything could happen between
>>> those syscalls.
>>
>> The design is:
>> 	sys_kexec_load()
>> 	shutdown scripts
>>         sys_reboot(LINUX_REBOOT_CMD_KEXEC);
>>
>> There are two system call simply so that the shutdown scripts can run.
>> Now maybe someone somewhere does something different but that is not
>> expected.

[...]

> Yes, and AFAIK, memory blocks which hold the reserved crashkernel area
> can usually not get offlined and, therefore, the memory cannot get removed.

The crashkernel area on arm64 will always land in un-removable memory. We set PG_Reserved
on it too.


Thanks,

James