Re: [PATCH v2 04/10] Introduce the CPU address space destruction function

[David Hildenbrand <david@redhat.com>]

On 14.09.23 15:00, lixianglai wrote:
> Hi David:

Hi!

> 
>> On 12.09.23 04:11, xianglai li wrote:
>>> Introduce new function to destroy CPU address space resources
>>> for cpu hot-(un)plug.
>>>
>> How do other archs handle that? Or how are they able to get away
>> without destroying?
>>
> They do not remove the cpu address space, taking the X86 architecture as
> an example:
> 
> 1.Start the x86 VM:
> 
> ./qemu-system-x86_64 \
> -machine q35  \
> -cpu Broadwell-IBRS \
> -smp 1,maxcpus=100,sockets=100,cores=1,threads=1 \
> -m 4G \
> -drive file=~/anolis-8.8.qcow2  \
> -serial stdio   \
> -monitor telnet:localhost:4498,server,nowait   \
> -nographic
> 
> 2.Connect the qemu monitor
> 
> telnet 127.0.0.1 4498
> 
> info mtree
> 
> address-space: cpu-memory-0
> address-space: memory
>     0000000000000000-ffffffffffffffff (prio 0, i/o): system
>       0000000000000000-000000007fffffff (prio 0, ram): alias ram-below-4g
> @pc.ram 0000000000000000-000000007fffffff
>       0000000000000000-ffffffffffffffff (prio -1, i/o): pci
>         00000000000a0000-00000000000bffff (prio 1, i/o): vga-lowmem
> 
> 3.Perform cpu hot swap int qemu monitor
> 
> device_add
> Broadwell-IBRS-x86_64-cpu,socket-id=1,core-id=0,thread-id=0,id=cpu1
> device_del cpu1
> 

Hm, doesn't seem to work for me on upstream QEMU for some reason: 
"Error: acpi: device unplug request for not supported device type: 
Broadwell-IBRS-x86_64-cpu"

What happens if you re-add that CPU? Will we reuse the previous address 
space?

> info mtree
> 
> address-space: cpu-memory-0
> address-space: cpu-memory-1
> address-space: memory
>     0000000000000000-ffffffffffffffff (prio 0, i/o): system
>       0000000000000000-000000007fffffff (prio 0, ram): alias ram-below-4g
> @pc.ram 0000000000000000-000000007fffffff
>       0000000000000000-ffffffffffffffff (prio -1, i/o): pci
>         00000000000a0000-00000000000bffff (prio 1, i/o): vga-lowmem
> 
> 
>   From the above test, you can see whether the address space of cpu1 is
> residual after a cpu hot swap, and whether it is reasonable?


Probably we should teach other archs to destroy that address space as well.

Can we do that from the core, instead of having to do that in each CPU 
unrealize function?

-- 
Cheers,

David / dhildenb