Good morning,
On 7/10/23 10:10, Steven Sistare wrote:
On 6/12/2023 10:59 AM, Michael Galaxy wrote:
Hi Steve,
On 6/7/23 12:37, Steven Sistare wrote:
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.
OK, so it is cpr-exec that needs this option (because of ram block allocation),
not really cpr-reboot.
Could the option then be made to only be required for cpr-exec and not
cpr-reboot, then,
since cpr-reboot doesn't require that consideration?
In a different forum Juan said this is a memory issue, so it should be expressed
as a memory related option. So, I will delete -migrate-enable-mode and revert
back
to -machine memfd-alloc, as defined in the V7 patch series.
Acknowledged. I'm going to try to get my reviewed-by's in soon. Sorry I
haven't done it sooner.
We've finished testing these patches on our systems and are moving forward.
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.
Still, only-cpr-capable is also optional. If and only if one needs this option,
the mode could be
specified as part of the option itself, rather than requiring an extra command
line parameter, no?
Yes, I will make that change.
- Steve
Acknowledged.
