Re: [PATCH V9 00/46] Live Update

[Steven Sistare]

On 6/7/2023 11:55 AM, Michael Galaxy wrote:
> Another option could be to expose "-migrate-mode-disable" (instead of enable) 
> and just enable all 3 modes by default,
> since we are already required to switch from "normal" mode to a CPR-specific 
> mode when it is time to do a live update,
> if the intention is to preserve the capability to completely prevent a 
> running QEMU from using these modes
> before the VM starts up.
> 
> - Michael
> 
> On 6/6/23 17:15, Michael Galaxy wrote:
>> Hi Steve,
>>
>> In the current design you have, we have to specify both the command line 
>> parameter "-migrate-mode-enable cpr-reboot"
>> *and* issue the monitor command "migrate_set_parameter mode cpr-${mode}".
>>
>> Is it possible to opt-in to the CPR mode just once over the monitor instead 
>> of having to specify it twice on the command line?
>> This would also match the live migration model: You do not need to 
>> necessarily "opt in" to live migration mode through
>> a command line parameter, you simply request it when you need to. Can CPR 
>> behave the same way?
>>
>> This would also make switching over to a CPR-capable version of QEMU much 
>> simpler and would even make it work for
>> existing libvirt-managed guests as their command line parameters would no 
>> longer need to change. This would allow us to
>> simply power-off and power-on existing VMs to make them CPR-capable and then 
>> work on a libvirt patch later when
>> we're ready to do so.
>>
>>
>> Comments?

Hi Michael,
  Requiring -migrate-enable-mode allows qemu to initialize objects
differently, if necessary, so that migration for a mode is not blocked.
See callers of migrate_mode_enabled.  There is only one so far, in
ram_block_add.  If the mode is cpr-exec, then it creates anonymous ram
blocks using memfd_create, else using MAP_ANON.  In the V7 series, this
was controlled by a '-machine memfd-alloc=on' option.  

migrate-enable-mode is more future proof for the user.  If something new must
initialize differently to support cpr, then it adds a call to 
migrate_mode_enabled,
and the command line remains the same.  However, I could be persuaded to go 
either way.

A secondary reason for -migrate-enable-mode is to support the only-cpr-capable
option.  It needs to know which mode will be used, in order to check a
mode-specific blocker list.

- Steve