Re: [PATCH V1 15/32] vl: QEMU_START_FREEZE env var

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

On 9/11/2020 2:49 PM, Dr. David Alan Gilbert wrote:
> * Steve Sistare (steven.sistare@oracle.com) wrote:
>> For qemu upgrade and restart, we will re-exec() qemu with the same argv.
>> However, qemu must start in a paused state and wait for the cprload command,
>> and the original argv might not contain the -S option.  To avoid modifying
>> argv, provide the QEMU_START_FREEZE environment variable.  If
>> QEMU_START_FREEZE is set, then set autostart=0, like the -S option.
>>
>> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> 
> What's wrong with modifying the argv?
> 
> Note, also the trick -incoming defer uses;  the whole point here is that
> we start qemu with   -incoming defer     and then we can issue commands
> to modify the QEMU configuration before we actually reload state.
> 
> Note, even without CPR there might be reasons that you need to modify
> the argv; for example, imagine that since it was originally booted
> someone had hotplug added an extra CPU or RAM or a disk; the new QEMU
> must be started in a state that reflects the state in which the VM was
> at the point when it was saved, not the point at which it was started
> long ago.

The code is simpler if we do not need to parse and massage the argv, and that is 
sufficient for many use cases.  QEMU_START_FREEZE adds only a few lines of code, and 
it's nice to have that choice.

For hot plug, we rely on the management layer to know what devices were plugged
after the initial startup, and re-plug them after restart.  cprsave restarts qemu,
which creates command-line devices.  At this point the manager would send the hotplug 
commands (just like -incoming defer), then send cprload. 

Having said that, if the management layer sometimes performs live migration, and sometimes
performs cpr restart, then we need to strip out any -incoming args from argv before restart.
This can be done in the vendor-specific qemu-exec helper (patch 20).

- Steve

>> ---
>>  softmmu/vl.c | 5 +++++
>>  1 file changed, 5 insertions(+)
>>
>> diff --git a/softmmu/vl.c b/softmmu/vl.c
>> index 951994f..7016e39 100644
>> --- a/softmmu/vl.c
>> +++ b/softmmu/vl.c
>> @@ -4501,6 +4501,11 @@ void qemu_init(int argc, char **argv, char **envp)
>>          exit(0);
>>      }
>>  
>> +    if (getenv("QEMU_START_FREEZE")) {
>> +        unsetenv("QEMU_START_FREEZE");
>> +        autostart = 0;
>> +    }
>> +
>>      if (incoming) {
>>          Error *local_err = NULL;
>>          qemu_start_incoming_migration(incoming, &local_err);
>> -- 
>> 1.8.3.1
>>