On Tuesday, January 30, 2018 5:25 PM, Peter Xu <pet...@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
