Re: [PATCH 00/10] Introduce virtio-mem <memory/> model

3 Feb 2021

      I did some test for virtio-mem with libvirt upstream version
v7.0.0-153-g5ea3ecd07d
 & qemu-kvm-5.2.0-0.7.rc2.fc34.x86_64

S1. Start domain with memory device

1. Domain configuration-
<maxMemory slots='16' unit='KiB'>10485760</maxMemory>
  <memory unit='KiB'>1572864</memory>
  <currentMemory unit='KiB'>1572864</currentMemory>
 ...
  <cpu mode='host-model' check='partial'>
    <numa>
      <cell id='0' cpus='0-3' memory='1048576' unit='KiB'/>
    </numa>
  </cpu>
...
  <memory model='virtio-mem'>
      <target>
        <size unit='KiB'>524288</size>
        <node>0</node>
        <block unit='KiB'>2048</block>
        <requested unit='KiB'>393216</requested>
      </target>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0a'
function='0x0'/>
    </memory>
#virsh start pc
 Domain 'pc'  started

2. The domain is started and check mem status, the actual size is "1048576"
#virsh dommemstat pc
actual 1048576
swap_in 0
swap_out 0
major_fault 257
minor_fault 130540
unused 604064
available 761328
usable 578428
last_update 1612325471
disk_caches 49632
hugetlb_pgalloc 0
hugetlb_pgfail 0
rss 460260

3. Then, check the active xml -
# virsh dumpxml pc
...
<memory unit='KiB'>1572864</memory>
  <currentMemory unit='KiB'>1048576</currentMemory>
....
 <memory model='virtio-mem'>
      <target>
        <size unit='KiB'>524288</size>
        <node>0</node>
        <block unit='KiB'>2048</block>
        <requested unit='KiB'>393216</requested>
        <actual unit='KiB'>0</actual>
      </target>
      <alias name='virtiomem0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0a'
function='0x0'/>
    </memory>

 Question1 : the value of actual is "0". Is it expected?

S2. Also, tried to use hugepage to start the domain -

<memory model='virtio-mem'>
      <target>
        <size unit='KiB'>524288</size>
        <node>0</node>
        <block unit='KiB'>2048</block>
        <requested unit='KiB'>393216</requested>
        <actual unit='KiB'>0</actual>
      </target>
      <alias name='virtiomem0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0a'
function='0x0'/>
    </memory>

#virsh start pc
error: Failed to start domain 'pc'
error: internal error: process exited while connecting to monitor:
2021-02-03T05:50:33.157836Z qemu-system-x86_64: -object
memory-backend-file,id=memvirtiomem0,mem-path=/dev/hugepages/libvirt/qemu/9-pc,size=536870912,host-nodes=0,policy=bind:
can't open backing store /dev/hugepages/libvirt/qemu/9-pc for guest RAM:
Permission denied

Question 2: any bug here?

On Tue, Feb 2, 2021 at 9:44 PM Peter Krempa <pkrempa@redhat.com> wrote:
...
On Fri, Jan 22, 2021 at 13:50:21 +0100, Michal Privoznik wrote:
...
Technically, this is another version of:
https://www.redhat.com/archives/libvir-list/2020-December/msg00199.html
But since virtio-pmem part is pushed now, I've reworked virtio-mem a bit
and sending it as a new series.
For curious ones, David summarized behaviour well when implementing
virtio-mem support in kernel:
https://lwn.net/Articles/755423/
For less curious ones:
# virsh update-memory $dom --requested-size 4G
adds additional 4GiB of RAM to guest;
# virsh update-memory $dom --requested-size 0
removes those 4GiB added earlier.
Patches are also available on my GitLab:
https://gitlab.com/MichalPrivoznik/libvirt/-/tree/virtio_mem_v3
Patches 1-7,9-10 (but observe some individual comments, including
the rename of the virsh commands):
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Patch 8 has severe semantic problems.
-- 
Thanks & Regards,
Jing,Qi

Re: [PATCH 00/10] Introduce virtio-mem <memory/> model

Jing Qi