Re: [Qemu-devel] Savevm and loadvm not working

[Peter Xu <peterx@redhat.com>]

On Tue, Jan 30, 2018 at 10:34:31AM +0000, sridhar kulkarni via Qemu-devel wrote:
> Hi,
> I am new bee to snapshot feature and how to use it correctly. My requirement is simple, in that I want to snapshot the running VM and save the snapshot file. Using the snapshot file, I want to boot the VM directly to snapshot state.I came across the qemu monitors "savevm" and "loadvm" commands. The following are the steps I follow to create snapshot and load the snapshot,Step 1: Launch VM using below command, 
> qemu-system-arm -M arm-machine -m 512M -name my_name -kernel main.rbx -serial pty -serial vc -serial vc -serial vc -drive if=none,format=qcow2,file=/home/sridhar/qemu_disk_image/dummy.qcow2 
> 
> Step 2: Save the snapshot (issue savevm from qemu monitor)
>         --> savevm my_snapshot
> 
> Step 3 Launch the VM using snapshot image saved in step 2,
> qemu-system-arm -M arm-machine -m 512M -name my_name -kernel main.rbx -serial pty -serial vc -serial vc -serial vc -drive if=none,format=qcow2,file=/home/sridhar/qemu_disk_image/dummy.qcow2 -loadvm my_snapshot
> With this approach, I am seeing that RAM contents are not getting saved when I issue "savevm" command.

Could I ask how do you know that RAM contents are not saved?  Is there
any error happened after your loadvm operation?

> I have copied the part of the file, when the function "ram_control_save_page" returns "RAM_SAVE_CONTROL_NOT_SUPP" and hence the page is not getting saved.
> 
> size_t ram_control_save_page(QEMUFile *f, ram_addr_t block_offset,
>                              ram_addr_t offset, size_t size,
>                              uint64_t *bytes_sent)
> {
>     if (f->hooks && f->hooks->save_page) {
>         int ret = f->hooks->save_page(f, f->opaque, block_offset,
>                                       offset, size, bytes_sent);
> 
>         if (ret != RAM_SAVE_CONTROL_DELAYED) {
>             if (bytes_sent && *bytes_sent > 0) {
>                 qemu_update_position(f, *bytes_sent);
>             } else if (ret < 0) {
>                 qemu_file_set_error(f, ret);
>             }
>         }
> 
>         return ret;
>     }
> 
>     return RAM_SAVE_CONTROL_NOT_SUPP;

Here IMHO as long as you are not using RDMA, this function should
always return with RAM_SAVE_CONTROL_NOT_SUPP.

And I do think the name is slightly misleading.

> }
> 
> Is there anything that I am missing here in the understanding "savevm" and "loadvm" commands? 
> 
> Thanks

-- 
Peter Xu