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Re: [Qemu-ppc] [RFC 0/4] target-ppc: handle KVM timebase migration
From: |
David Gibson |
Subject: |
Re: [Qemu-ppc] [RFC 0/4] target-ppc: handle KVM timebase migration |
Date: |
Fri, 15 Apr 2016 15:27:04 +1000 |
User-agent: |
Mutt/1.5.24 (2015-08-30) |
On Thu, Apr 07, 2016 at 04:23:10PM +0100, Mark Cave-Ayland wrote:
> This RFC series follows on from discussions in the "Migrating decrementer"
> thread relating to handling migration of the timebase and decrementer
> registers for both TCG and KVM. The aim is to provide a consistent virtual
> clock when migrating PPC machines TCG-TCG (already handled) and
> KVM-KVM with both paused and live VMs in a unified manner for all PPC
> machine types.
>
> The existing code to handle timebase migration on PPC has a few issues:
>
> 1) It uses cpu_get_host_ticks() which doesn't work correctly on TCG with
> different host architectures
Yeah, that's just wrong. We should be using qemu_clock_get_ns()
instead.
> 2) tb_offset is calculated from the difference in the host clock between
> source and destination. Thus if the guest is paused then the resulting
> offset is calculated incorrectly since the host clock still runs, regardless
> of the state of the virtual clock.
>
> 3) Due to a typo in the existing implementation (MIN instead of MAX) then
> guest_tb never takes migration_duration_tb into account
The statement above is not correct. The problem is not MIN vs. MAX.
The problem is that it was assuming a 1s limit on downtime during
migration which is Just Plain Wrong - especialy in the case where it's
actually a savevm / restore rather than a live migration.
Having a long downtime shouldn't break the guest's timebase sync with
realtime.
> Patches 1-2 introduce a new PPCMachineClass and derive all the PPC machine
> classes from it, not dissimilar to x86's PCMachineClass.
>
> Patch 3 adds a VM state change hook into PCMachineClass and updates the
> tb_offset from the host when the machine (re)starts as discussed in the
> thread. Note that it may be possible to add the hook to specific machine
> types, however using PPCMachineClass allows all PPC machines to pick up
> this functionality automatically.
>
> Patch 4 temporarily disables the existing timebase migration code to allow
> testing.
>
> Note that this has only been compile-tested on x86 as I have no KVM PPC
> hardware and so it's really an RFC to determine whether this approach can
> work, and to invite testing and discussion from existing KVM PPC users.
>
> Signed-off-by: Mark Cave-Ayland <address@hidden>
>
>
> Mark Cave-Ayland (4):
> target-ppc: introduce PPCMachineClass and PPCMachineState
> target-ppc: derive all PPC machine classes to have PPCMachineClass as
> their superclass
> target-ppc: synchronise tb_offset with KVM host on machine start
> target-ppc: hack to remove existing timebase migration code for
> testing
>
> hw/ppc/e500plat.c | 7 +++-
> hw/ppc/mac_newworld.c | 17 ++------
> hw/ppc/mac_oldworld.c | 6 ++-
> hw/ppc/mpc8544ds.c | 7 +++-
> hw/ppc/ppc.c | 100
> +++++++++++++++++++++++++++++------------------
> hw/ppc/ppc405_boards.c | 28 +++----------
> hw/ppc/ppc440_bamboo.c | 6 ++-
> hw/ppc/prep.c | 6 ++-
> hw/ppc/spapr.c | 2 +-
> hw/ppc/virtex_ml507.c | 6 ++-
> include/hw/ppc/ppc.h | 35 +++++++++++++++++
> include/qemu/typedefs.h | 2 +
> 12 files changed, 135 insertions(+), 87 deletions(-)
>
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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