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

[sridhar kulkarni <sridhar_kulk@yahoo.com>]

 

    On Tuesday, January 30, 2018 5:25 PM, Peter Xu <peterx@redhat.com> wrote:
 

 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 thought that RAM contents are not saved because of return value RAM_SAVE_CONTROL_NOT_SUPP. Now that you clarified about this, I debugged this further. After doing loadvm operation, I am getting following error,qemu-system-arm: error while loading state for instance 0x0 of device 'cpu'
qemu-system-arm: Error -1 while loading VM state
I figured out that, this error is because of following function returning FALSE. "write_raw_cp_reg" writes a value and "read_raw_cp_reg" reads the value back. There is mismatch between what we write and what is being read back, and hence function returns FALSE.
Between I want to mention that, I am trying the snapshot on a VM which runs ARM 7 core. I am not sure if this snapshot feature is fully function for ARM targets.

bool write_list_to_cpustate(ARMCPU *cpu)
{
    int i;
    bool ok = true;

    for (i = 0; i < cpu->cpreg_array_len; i++) {
        uint32_t regidx = kvm_to_cpreg_id(cpu->cpreg_indexes[i]);
        uint64_t v = cpu->cpreg_values[i];
        const ARMCPRegInfo *ri;

        ri = get_arm_cp_reginfo(cpu->cp_regs, regidx);
        if (!ri) {
            ok = false;
            continue;
        }
        if (ri->type & ARM_CP_NO_RAW) {
            continue;
        }
        /* Write value and confirm it reads back as written
         * (to catch read-only registers and partially read-only
         * registers where the incoming migration value doesn't match)
         */
        write_raw_cp_reg(&cpu->env, ri, v);
        if (read_raw_cp_reg(&cpu->env, ri) != v) {
            ok = false;
        }
    }
    return ok;

> 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