On Fri, Sep 11, 2026 at 04:23:43PM +0200, Denis V. Lunev wrote: > Restoring a big Windows guest spends most of its destination-side time in > post_load hooks which do nothing but move memory regions around. Each one > ends a memory transaction, and a transaction commit re-renders every > flatview it touches at a cost which grows with the number of regions in > the machine. A hook which runs once per vCPU therefore pays that render > once per vCPU, and the machine gets slower to migrate the bigger it is. > > The Hyper-V SynIC is the case that hurts: restoring the synthetic > interrupt controller maps a message page and an event page per vCPU, so a > 64-vCPU guest forces 128 remaps, each with its own rebuild, while the rest > of the stream is still being read.
This is partly a known issue, not from Hyper-V, but from virtio mmio regions.. please see: https://wiki.qemu.org/ToDo/LiveMigration#Optimize_memory_updates_for_non-iterative_vmstates https://lore.kernel.org/r/[email protected] I believe we also thought about do MR update per-device, so batching but smaller scale, easier to make sure no illegal access to a stale flatview. So in general, I agree this approach might be the right way to do, which is to shrink the transaction to be smaller than "batch everything".. as what Chuang used to do. I still have some pure questions inline. > > Patch 1 adds post_load_deferrable. A vmsd which sets it has its hook Nit, IMHO if so it needs to be called "deferred", as "deferrable" implies the defer is optional. > queued during the load and run once the stream has been consumed, in the > order the hooks would have fired, with the whole drain sharing one memory > transaction. Patch 2 sets it on the SynIC subsection. > > It is opt-in rather than automatic, and the three preconditions are > spelled out on the field: the hook must not fail, nothing later in the Actually, I _think_ maybe it can still fail.. IIUC source QEMU only dies if it receives shut from migrate_send_rp_shut(), which is after the deferred loads at least with the current change. Worth check.. > load may depend on what it does, and it must not read guest memory or > resolve an address space. Yes, but I think this is partial of the whole picture: IIUC if this deferred hook may inject some MRs that may be accessed by other VMSD loaders, or anything (including hard-coded loading process), I think it's an issue too. So personally I don't like this API very much yet on how it was defined; it'll be very hard to be used right unless we fully understand what will happen.. I wonder if there's better way to define the API to be clearer. Since all the known issues about this is about MR updates: virtio MMIO regions, hyper-v, pci bar/bridge (mentioned below), I wonder if this can be something dedicated to MR updates, and maybe it doesn't need to be "deferred", just grouped together properly into one transaction, which can happen in the middle too or maybe it doesn't matter much. Then it applies some form of limitation to what can be split out from normal VMSD flow. The current API relies on allowing to defer anything, which is fine but very hard to control, and we may face tricky bugs if users grows but when they're not used right.. > A hook which breaks the first is fatal rather > than silently reported, because by drain time the source may already have > been told the migration succeeded. > > Deferring is not free in general, which is the other reason it is opt-in. > Deferring the APIC post_load, whose cost is a synchronous run_on_cpu per > vCPU rather than a memory remap, moves 3 ms out of the section walk and > pays about 9 ms of drain for it. Deferral helps a hook which repeats I'm just curious: why something will take 9ms if deferred, even if it used to take 3ms? I think I misread something, but I can't tell myself. > topology work; it makes a hook which does cross-thread work worse. > > Measurements > ------------ > > Destination-side non-iterable load, ie. the sum of vmstate_downtime_load > over non-iterable sections, on a guest which has actually programmed its > Hyper-V state. Five interleaved rounds per point on an otherwise idle > host, twice; medians, with the spread across all ten rounds. > > upstream 377 ms (363-400) > + pci mapping transactions 247 ms (242-255) > + this series 96 ms (86-97) Definitely a great improvement. I think we need this, just one way or another. I may have some other trivial comments later in the patch. Thanks for working on it. > > Two postings against one problem, so the whole ladder is shown. The first > step is a pci pair which batches a device's BAR and bridge window updates > into a single transaction, posted separately and now queued in Michael's > tree: > > https://lore.kernel.org/qemu-devel/[email protected]/ > > Those two are listed because they change what a rebuild costs, and so > change what this series is worth. Together the postings take the load > from 377 ms to 96 ms; this series is the 247 ms to 96 ms step. > > Where it goes: the cpu sections fall from 154 ms to 1.3 ms. The deferred > hooks themselves cost 77 us at the drain, so the work is removed rather > than moved somewhere the per-section metric cannot see. > > The saving scales with vCPU count, since that is how many times the remap > repeats, and with the number of memory regions in the machine, since that > is what a rebuild costs. > > Guest under test > ---------------- > > Windows Server 2022, installed unattended, idle at the console: > > -machine q35,accel=kvm > -cpu host,hv-synic,hv-stimer,hv-stimer-direct,hv-vapic,hv-runtime, > hv-time,hv-ipi,hv-crash,hv-reset,hv-frequencies,hv-vpindex, > hv-spinlocks=0x1fff > -smp 64,sockets=2,cores=32,threads=1 > -m 4G > 65 pcie-root-ports, 9 virtio devices behind them, qxl > > Host: AMD EPYC 7443P, 24 cores / 48 threads. > > hv-synic is the flag that matters. Without it the guest never programs > the SynIC pages and the effect under test does not exist. > > Measured with a save/restore harness rather than a live migration: a > restore from a captured stream walks the same qemu_loadvm_state_main() > path a destination does, which removes libvirt, the network and the > second host from the measurement. > > CC: Peter Xu <[email protected]> > CC: Fabiano Rosas <[email protected]> > CC: Paolo Bonzini <[email protected]> > CC: Zhao Liu <[email protected]> > Signed-off-by: Denis V. Lunev <[email protected]> > > Denis V. Lunev (2): > migration: let a vmstate defer its post_load to end of stream > target/i386: defer the Hyper-V SynIC post_load > > include/migration/vmstate.h | 40 +++++++++++++ > migration/savevm.c | 23 ++++++++ > migration/vmstate.c | 67 +++++++++++++++++++++- > target/i386/machine.c | 1 + > tests/unit/test-vmstate.c | 111 ++++++++++++++++++++++++++++++++++++ > 5 files changed, 240 insertions(+), 2 deletions(-) > > -- > 2.53.0 > -- Peter Xu
