Re: [PATCH RFCv3 9/9] s390x: initial support for virtio-mem

[David Hildenbrand]

On 27.07.20 12:03, Cornelia Huck wrote:
> On Fri, 24 Jul 2020 16:37:50 +0200
> David Hildenbrand <david@redhat.com> wrote:
> 
>> Let's wire up the initial, basic virtio-mem implementation in QEMU. It will
>> have to see some important extensions (esp., resizeable allocations)
>> before it can be considered production ready. Also, the focus on the Linux
>> driver side is on memory hotplug, there are a lot of things optimize in
>> the future to improve memory unplug capabilities. However, the basics
>> are in place.
>>
>> Block migration for now, as we'll have to take proper care of storage
>> keys and storage attributes. Also, make sure to not hotplug huge pages
>> to a setup without huge pages.
>>
>> With a Linux guest that supports virtio-mem (and has
>> CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE set for now), a basic example.
>>
>> 1. Start a VM with 2G initial memory and a virtio-mem device with a maximum
>>    capacity of 18GB (and an initial size of 300M):
>>     sudo qemu-system-s390x \
>>         --enable-kvm \
>>         -m 2G,maxmem=20G \
>>         -smp 4 \
>>         -nographic \
>>         -chardev socket,id=monitor,path=/var/tmp/monitor,server,nowait \
>>         -mon chardev=monitor,mode=readline \
>>         -net nic -net user \
>>         -hda s390x.cow2 \
>>         -object memory-backend-ram,id=mem0,size=18G \
>>         -device virtio-mem-ccw,id=vm0,memdev=mem0,requested-size=300M
>>
>> 2. Query the current size of virtio-mem device:
>>     (qemu) info memory-devices
>>     Memory device [virtio-mem]: "vm0"
>>       memaddr: 0x80000000
>>       node: 0
>>       requested-size: 314572800
>>       size: 314572800
>>       max-size: 19327352832
>>       block-size: 1048576
>>       memdev: /objects/mem0
>>
>> 3. Request to grow it to 8GB:
>>     (qemu) qom-set vm0 requested-size 8G
>>     (qemu) info memory-devices
>>     Memory device [virtio-mem]: "vm0"
>>       memaddr: 0x80000000
>>       node: 0
>>       requested-size: 8589934592
>>       size: 8589934592
>>       max-size: 19327352832
>>       block-size: 1048576
>>       memdev: /objects/mem0
>>
>> 4. Request to shrink it to 800M (might take a while, might not fully
>>    succeed, and might not be able to remove memory blocks in Linux):
>>   (qemu) qom-set vm0 requested-size 800M
>>   (qemu) info memory-devices
>>   Memory device [virtio-mem]: "vm0"
>>     memaddr: 0x80000000
>>     node: 0
>>     requested-size: 838860800
>>     size: 838860800
>>     max-size: 19327352832
>>     block-size: 1048576
>>     memdev: /objects/mem0
>>
>> Note 1: Due to lack of resizeable allocations, we will go ahead and
>> reserve a 18GB vmalloc area + size the QEMU RAM slot + KVM mamory slot
>> 18GB. echo 1 > /proc/sys/vm/overcommit_memory might be required for
>> now. In the future, this area will instead grow on actual demand and shrink
>> when possible.
>>
>> Note 2: Although virtio-mem-pci is wired up as well, it does not seem to
>> work currently on s390x due to lack of MSI-X.
> 
> IIRC, you can trick virtio-pci into using msi-x via nvectors. Might be
> interesting to try.

Ah, okay, I tried to find an option to enable it "msi-x=on" but failed.
Will give it a try. Thanks!


-- 
Thanks,

David / dhildenb