Re: [PATCH V3 00/22] Live Update [restart] : code replication

[Steven Sistare <steven.sistare@oracle.com>]

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On 6/3/2021 3:36 PM, Dr. David Alan Gilbert wrote:
> * Steven Sistare (steven.sistare@oracle.com) wrote:
>> On 5/24/2021 6:39 AM, Dr. David Alan Gilbert wrote:
>>> * Steven Sistare (steven.sistare@oracle.com) wrote:
>>>> On 5/20/2021 9:13 AM, Dr. David Alan Gilbert wrote:
>>>>> On the 'restart' branch of questions; can you explain,
>>>>> other than the passing of the fd's, why the outgoing side of
>>>>> qemu's 'migrate exec:' doesn't work for you?
>>>>
>>>> I'm not sure what I should describe.  Can you be more specific?
>>>> Do you mean: can we add the cpr specific bits to the migrate exec code?
>>>
>>> Yes; if possible I'd prefer to just keep the one exec mechanism.
>>> It has an advantage of letting you specify the new command line; that
>>> avoids the problems I'd pointed out with needing to change the command
>>> line if a hotplug had happened.  It also means we only need one chunk of
>>> exec code.
>>
>> [...]
> 
> I'm not quite sure what I want in the incoming there, but that is
> already the source execing the destination qemu - although I think we'd
> probably need to check if that's actually via an intermediary.
> 
>> We could shoehorn cpr restart into the migrate exec path by defining a new migration 
>> capability that the client would set before issuing the migrate command.  However, the
>> implementation code would be sprinkled with conditionals to suppress migrate-only bits
>> and call cpr-only bits.  IMO that would be less maintainable than having a separate
>> cprsave function.  Currently cprsave does not duplicate any migration functionality.
>> cprsave calls qemu_save_device_state() which is used by xen.
> 
> To me it feels like cprsave in particular is replicating more code. 

In the attached file I annotated lines of code that have some overlap
with migration code actions.  They include vm control, global_state_store,
and vmstate save, and cover 18 lines of 78 total.  I did not include the
body of qf_file_open because it is also called by xen.

The migration code adds capabilities, parameters, state, status, info,
precopy, postcopy, dirty bitmap, events, notifiers, 6 channel types,
blockers, pause+resume, return path, request-reply commands, throttling, colo,
blocks, phases, iteration, and threading, implemented by 20000+ lines of code.
To me it seems wrong to throw cpr into that mix to avoid adding tens of lines 
of similar code.

- Steve

[-- Attachment #2: cprsave.txt --]
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  void cprsave(const char *file, CprMode mode, Error **errp)
  {
      int ret = 0;
**    QEMUFile *f;
**    int saved_vm_running = runstate_is_running();
      bool restart = (mode == CPR_MODE_RESTART);
      bool reboot = (mode == CPR_MODE_REBOOT);
  
      if (reboot && qemu_ram_volatile(errp)) {
          return;
      }
  
      if (restart && xen_enabled()) {
          error_setg(errp, "xen does not support cprsave restart");
          return;
      }
  
      if (migrate_colo_enabled()) {
          error_setg(errp, "error: cprsave does not support x-colo");
          return;
      }
  
      if (replay_mode != REPLAY_MODE_NONE) {
          error_setg(errp, "error: cprsave does not support replay");
          return;
      }
  
      f = qf_file_open(file, O_CREAT | O_WRONLY | O_TRUNC, 0600, "cprsave", errp);
      if (!f) {
          return;
      }
  
**    ret = global_state_store();
**    if (ret) {
**        error_setg(errp, "Error saving global state");
**        qemu_fclose(f);
**        return;
**    }
      if (runstate_check(RUN_STATE_SUSPENDED)) {
          /* Update timers_state before saving.  Suspend did not so do. */
          cpu_disable_ticks();
      }
**    vm_stop(RUN_STATE_SAVE_VM);
  
      cpr_is_active = true;
**    ret = qemu_save_device_state(f);
**    qemu_fclose(f);
**    if (ret < 0) {
**        error_setg(errp, "Error %d while saving VM state", ret);
**        goto err;
**    }
  
      if (reboot) {
          shutdown_action = SHUTDOWN_ACTION_POWEROFF;
          qemu_system_shutdown_request(SHUTDOWN_CAUSE_GUEST_SHUTDOWN);
      } else if (restart) {
          if (!qemu_chr_cpr_capable(errp)) {
              goto err;
          }
          if (vfio_cprsave(errp)) {
              goto err;
          }
          walkenv(FD_PREFIX, preserve_fd, 0);
          vhost_dev_reset_all();
          qemu_term_exit();
          setenv("QEMU_START_FREEZE", "", 1);
          qemu_system_exec_request();
      }
      goto done;
  
  err:
**    if (saved_vm_running) {
**        vm_start();
**    }
  done:
      cpr_is_active = false;
      return;
  }