Re: [PATCH][RESEND v5 3/3] Add a Hyper-V Dynamic Memory Protocol driver (hv-balloon)

[David Hildenbrand <david@redhat.com>]

[...]

>>>> We'd use a memory region container as device memory region (like [1]) and would have to handle the !memdev case (I can help with that). > Into that, you can map the RAM memory region on demand (and eventually even using multiple slots like [1]).
>>>>
>>>> (2) Use a single virtual DIMM and (un)plug that on demand. Let the machine code handle (un)plugging of the device.
>>>>
>>>>
>>>> (1) feels cleanest to me, although it will require a bit more work.
>>>>
>>>
>>> I also think approach (1) makes more sense as it avoids memslot metadata
>>> overhead for not-yet-hot-added parts of the memory backing device.
>>>
>>> Not sure what you mean that the !memdev case would be problematic in this
>>> case - it is working in the current driver shape so why would adding
>>> potential memory subregions (used in the memdev case) change that?
>>
>> I'm thinking about the case where you have a hv-balloon device without a memdev.
>>
>> Without -m X,maxmem=y we don't currently expect to have memory devices around
>> (and especially them getting (un)plugged. But why should we "force" to set the
>> "maxmem" option
> 
> I guess it's only a small change to QEMU to allow having hv-balloon
> device (without a memdev) even in the case where there's no "maxmem"
> option given on the QEMU command line.
> 
>>
>> I hope I'll find some time soonish to prototype what I have in mind, to see
>> if it could be made working.
>>
> 
> Okay, so I'll wait for your prototype before commencing further work on
> the next version of this driver.

About to have something simplistic running -- I think. Want to test with 
a Linux VM, but I don't seem to get it working (also without my changes).


#!/bin/bash

build/qemu-system-x86_64 \
     --enable-kvm \
     -m 4G,maxmem=36G \
     -cpu host,hv-syndbg=on,hv-synic,hv-relaxed,hv-vpindex \
     -smp 16 \
     -nographic \
     -nodefaults \
     -net nic -net user \
     -chardev stdio,nosignal,id=serial \
     -hda Fedora-Cloud-Base-37-1.7.x86_64.qcow2 \
     -cdrom /home/dhildenb/git/cloud-init/cloud-init.iso \
     -device isa-serial,chardev=serial \
     -chardev socket,id=monitor,path=/var/tmp/mon_src,server,nowait \
     -mon chardev=monitor,mode=readline \
     -device vmbus-bridge \
     -object memory-backend-ram,size=2G,id=mem0 \
     -device hv-balloon,id=hv1,memdev=mem0



[root@vm-0 ~]# uname -r
6.3.5-100.fc37.x86_64
[root@vm-0 ~]# modprobe hv_balloon
modprobe: ERROR: could not insert 'hv_balloon': No such device


Any magic flag I am missing? Or is there something preventing this to 
work with Linux VMs?

-- 
Cheers,

David / dhildenb