On Wed, Aug 27, 2025 at 6:56 PM Jonah Palmer <[email protected]> wrote: > > > > On 8/20/25 3:59 AM, Eugenio Perez Martin wrote: > > On Tue, Aug 19, 2025 at 5:11 PM Jonah Palmer <[email protected]> > > wrote: > >> > >> > >> > >> On 8/19/25 3:10 AM, Eugenio Perez Martin wrote: > >>> On Mon, Aug 18, 2025 at 4:46 PM Jonah Palmer <[email protected]> > >>> wrote: > >>>> > >>>> > >>>> > >>>> On 8/18/25 2:51 AM, Eugenio Perez Martin wrote: > >>>>> On Fri, Aug 15, 2025 at 4:50 PM Jonah Palmer <[email protected]> > >>>>> wrote: > >>>>>> > >>>>>> > >>>>>> > >>>>>> On 8/14/25 5:28 AM, Eugenio Perez Martin wrote: > >>>>>>> On Wed, Aug 13, 2025 at 4:06 PM Peter Xu <[email protected]> wrote: > >>>>>>>> > >>>>>>>> On Wed, Aug 13, 2025 at 11:25:00AM +0200, Eugenio Perez Martin wrote: > >>>>>>>>> On Mon, Aug 11, 2025 at 11:56 PM Peter Xu <[email protected]> wrote: > >>>>>>>>>> > >>>>>>>>>> On Mon, Aug 11, 2025 at 05:26:05PM -0400, Jonah Palmer wrote: > >>>>>>>>>>> This effort was started to reduce the guest visible downtime by > >>>>>>>>>>> virtio-net/vhost-net/vhost-vDPA during live migration, especially > >>>>>>>>>>> vhost-vDPA. > >>>>>>>>>>> > >>>>>>>>>>> The downtime contributed by vhost-vDPA, for example, is not from > >>>>>>>>>>> having to > >>>>>>>>>>> migrate a lot of state but rather expensive backend control-plane > >>>>>>>>>>> latency > >>>>>>>>>>> like CVQ configurations (e.g. MQ queue pairs, RSS, MAC/VLAN > >>>>>>>>>>> filters, offload > >>>>>>>>>>> settings, MTU, etc.). Doing this requires kernel/HW NIC > >>>>>>>>>>> operations which > >>>>>>>>>>> dominates its downtime. > >>>>>>>>>>> > >>>>>>>>>>> In other words, by migrating the state of virtio-net early > >>>>>>>>>>> (before the > >>>>>>>>>>> stop-and-copy phase), we can also start staging backend > >>>>>>>>>>> configurations, > >>>>>>>>>>> which is the main contributor of downtime when migrating a > >>>>>>>>>>> vhost-vDPA > >>>>>>>>>>> device. > >>>>>>>>>>> > >>>>>>>>>>> I apologize if this series gives the impression that we're > >>>>>>>>>>> migrating a lot > >>>>>>>>>>> of data here. It's more along the lines of moving control-plane > >>>>>>>>>>> latency out > >>>>>>>>>>> of the stop-and-copy phase. > >>>>>>>>>> > >>>>>>>>>> I see, thanks. > >>>>>>>>>> > >>>>>>>>>> Please add these into the cover letter of the next post. IMHO it's > >>>>>>>>>> extremely important information to explain the real goal of this > >>>>>>>>>> work. I > >>>>>>>>>> bet it is not expected for most people when reading the current > >>>>>>>>>> cover > >>>>>>>>>> letter. > >>>>>>>>>> > >>>>>>>>>> Then it could have nothing to do with iterative phase, am I right? > >>>>>>>>>> > >>>>>>>>>> What are the data needed for the dest QEMU to start staging backend > >>>>>>>>>> configurations to the HWs underneath? Does dest QEMU already have > >>>>>>>>>> them in > >>>>>>>>>> the cmdlines? > >>>>>>>>>> > >>>>>>>>>> Asking this because I want to know whether it can be done > >>>>>>>>>> completely > >>>>>>>>>> without src QEMU at all, e.g. when dest QEMU starts. > >>>>>>>>>> > >>>>>>>>>> If src QEMU's data is still needed, please also first consider > >>>>>>>>>> providing > >>>>>>>>>> such facility using an "early VMSD" if it is ever possible: feel > >>>>>>>>>> free to > >>>>>>>>>> refer to commit 3b95a71b22827d26178. > >>>>>>>>>> > >>>>>>>>> > >>>>>>>>> While it works for this series, it does not allow to resend the > >>>>>>>>> state > >>>>>>>>> when the src device changes. For example, if the number of > >>>>>>>>> virtqueues > >>>>>>>>> is modified. > >>>>>>>> > >>>>>>>> Some explanation on "how sync number of vqueues helps downtime" > >>>>>>>> would help. > >>>>>>>> Not "it might preheat things", but exactly why, and how that differs > >>>>>>>> when > >>>>>>>> it's pure software, and when hardware will be involved. > >>>>>>>> > >>>>>>> > >>>>>>> By nvidia engineers to configure vqs (number, size, RSS, etc) takes > >>>>>>> about ~200ms: > >>>>>>> https://urldefense.com/v3/__https://lore.kernel.org/qemu-devel/[email protected]/T/__;!!ACWV5N9M2RV99hQ!OQdf7sGaBlbXhcFHX7AC7HgYxvFljgwWlIgJCvMgWwFvPqMrAMbWqf0862zV5shIjaUvlrk54fLTK6uo2pA$ > >>>>>>> > >>>>>>> Adding Dragos here in case he can provide more details. Maybe the > >>>>>>> numbers have changed though. > >>>>>>> > >>>>>>> And I guess the difference with pure SW will always come down to PCI > >>>>>>> communications, which assume it is slower than configuring the host SW > >>>>>>> device in RAM or even CPU cache. But I admin that proper profiling is > >>>>>>> needed before making those claims. > >>>>>>> > >>>>>>> Jonah, can you print the time it takes to configure the vDPA device > >>>>>>> with traces vs the time it takes to enable the dataplane of the > >>>>>>> device? So we can get an idea of how much time we save with this. > >>>>>>> > >>>>>> > >>>>>> Let me know if this isn't what you're looking for. > >>>>>> > >>>>>> I'm assuming by "configuration time" you mean: > >>>>>> - Time from device startup (entry to vhost_vdpa_dev_start()) to > >>>>>> right > >>>>>> before we start enabling the vrings (e.g. > >>>>>> VHOST_VDPA_SET_VRING_ENABLE in vhost_vdpa_net_cvq_load()). > >>>>>> > >>>>>> And by "time taken to enable the dataplane" I'm assuming you mean: > >>>>>> - Time right before we start enabling the vrings (see above) to > >>>>>> right > >>>>>> after we enable the last vring (at the end of > >>>>>> vhost_vdpa_net_cvq_load()) > >>>>>> > >>>>>> Guest specs: 128G Mem, SVQ=on, CVQ=on, 8 queue pairs: > >>>>>> > >>>>>> -netdev type=vhost-vdpa,vhostdev=$VHOST_VDPA_0,id=vhost-vdpa0, > >>>>>> queues=8,x-svq=on > >>>>>> > >>>>>> -device virtio-net-pci,netdev=vhost-vdpa0,id=vdpa0,bootindex=-1, > >>>>>> romfile=,page-per-vq=on,mac=$VF1_MAC,ctrl_vq=on,mq=on, > >>>>>> ctrl_vlan=off,vectors=18,host_mtu=9000, > >>>>>> disable-legacy=on,disable-modern=off > >>>>>> > >>>>>> --- > >>>>>> > >>>>>> Configuration time: ~31s > >>>>>> Dataplane enable time: ~0.14ms > >>>>>> > >>>>> > >>>>> I was vague, but yes, that's representative enough! It would be more > >>>>> accurate if the configuration time ends by the time QEMU enables the > >>>>> first queue of the dataplane though. > >>>>> > >>>>> As Si-Wei mentions, is v->shared->listener_registered == true at the > >>>>> beginning of vhost_vdpa_dev_start? > >>>>> > >>>> > >>>> Ah, I also realized that Qemu I was using for measurements was using a > >>>> version before the listener_registered member was introduced. > >>>> > >>>> I retested with the latest changes in Qemu and set x-svq=off, e.g.: > >>>> guest specs: 128G Mem, SVQ=off, CVQ=on, 8 queue pairs. I ran testing 3 > >>>> times for measurements. > >>>> > >>>> v->shared->listener_registered == false at the beginning of > >>>> vhost_vdpa_dev_start(). > >>>> > >>> > >>> Let's move out the effect of the mem pinning from the downtime by > >>> registering the listener before the migration. Can you check why is it > >>> not registered at vhost_vdpa_set_owner? > >>> > >> > >> Sorry I was profiling improperly. The listener is registered at > >> vhost_vdpa_set_owner initially and v->shared->listener_registered is set > >> to true, but once we reach the first vhost_vdpa_dev_start call, it shows > >> as false and is re-registered later in the function. > >> > >> Should we always expect listener_registered == true at every > >> vhost_vdpa_dev_start call during startup? > > > > Yes, that leaves all the memory pinning time out of the downtime. > > > >> This is what I traced during > >> startup of a single guest (no migration). > > > > We can trace the destination's QEMU to be more accurate, but probably > > it makes no difference. > > > >> Tracepoint is right at the > >> start of the vhost_vdpa_dev_start function: > >> > >> vhost_vdpa_set_owner() - register memory listener > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 1, started = 0 > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > > > > This is surprising. Can you trace how listener_registered goes to 0 again? > > > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > >> ... > >> * VQs are now being enabled * > >> > >> I'm also seeing that when the guest is being shutdown, > >> dev->vhost_ops->vhost_get_vring_base() is failing in > >> do_vhost_virtqueue_stop(): > >> > >> ... > >> [ 114.718429] systemd-shutdown[1]: Syncing filesystems and block devices. > >> [ 114.719255] systemd-shutdown[1]: Powering off. > >> [ 114.719916] sd 0:0:0:0: [sda] Synchronizing SCSI cache > >> [ 114.724826] ACPI: PM: Preparing to enter system sleep state S5 > >> [ 114.725593] reboot: Power down > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 1, started = 0 > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 1, started = 0 > >> qemu-system-x86_64: vhost VQ 2 ring restore failed: -1: Operation not > >> permitted (1) > >> qemu-system-x86_64: vhost VQ 3 ring restore failed: -1: Operation not > >> permitted (1) > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 1, started = 0 > >> qemu-system-x86_64: vhost VQ 4 ring restore failed: -1: Operation not > >> permitted (1) > >> qemu-system-x86_64: vhost VQ 5 ring restore failed: -1: Operation not > >> permitted (1) > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 1, started = 0 > >> qemu-system-x86_64: vhost VQ 6 ring restore failed: -1: Operation not > >> permitted (1) > >> qemu-system-x86_64: vhost VQ 7 ring restore failed: -1: Operation not > >> permitted (1) > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 1, started = 0 > >> qemu-system-x86_64: vhost VQ 8 ring restore failed: -1: Operation not > >> permitted (1) > >> qemu-system-x86_64: vhost VQ 9 ring restore failed: -1: Operation not > >> permitted (1) > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 1, started = 0 > >> qemu-system-x86_64: vhost VQ 10 ring restore failed: -1: Operation not > >> permitted (1) > >> qemu-system-x86_64: vhost VQ 11 ring restore failed: -1: Operation not > >> permitted (1) > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 1, started = 0 > >> qemu-system-x86_64: vhost VQ 12 ring restore failed: -1: Operation not > >> permitted (1) > >> qemu-system-x86_64: vhost VQ 13 ring restore failed: -1: Operation not > >> permitted (1) > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 1, started = 0 > >> qemu-system-x86_64: vhost VQ 14 ring restore failed: -1: Operation not > >> permitted (1) > >> qemu-system-x86_64: vhost VQ 15 ring restore failed: -1: Operation not > >> permitted (1) > >> vhost_vdpa_dev_start() - v->shared->listener_registered = 1, started = 0 > >> > >> However when x-svq=on, I don't see these errors on shutdown. > >> > > > > SVQ can mask this error as it does not need to forward the ring > > restore message to the device. It can just start with 0 and convert > > indexes. > > > > Let's focus on listened_registered first :). > > > >>>> --- > >>>> > >>>> Configuration time: Time from first entry into vhost_vdpa_dev_start() to > >>>> right after Qemu enables the first VQ. > >>>> - 26.947s, 26.606s, 27.326s > >>>> > >>>> Enable dataplane: Time from right after first VQ is enabled to right > >>>> after the last VQ is enabled. > >>>> - 0.081ms, 0.081ms, 0.079ms > >>>> > >>> > >> > > > > I looked into this a bit more and realized I was being naive thinking > that the vhost-vDPA device startup path of a single VM would be the same > as that on a destination VM during live migration. This is **not** the > case and I apologize for the confusion I caused. > > What I described and profiled above is indeed true for the startup of a > single VM / source VM with a vhost-vDPA device. However, this is not > true on the destination side and its configuration time is drastically > different. > > Under the same specs, but now with a live migration performed between a > source and destination VM (128G Mem, SVQ=off, CVQ=on, 8 queue pairs), > and using the same tracepoints to find the configuration time and enable > dataplane time, these are the measurements I found for the **destination > VM**: > > Configuration time: Time from first entry into vhost_vdpa_dev_start to > right after Qemu enables the first VQ. > - 268.603ms, 241.515ms, 249.007ms > > Enable dataplane time: Time from right after the first VQ is enabled to > right after the last VQ is enabled. > - 0.072ms, 0.071ms, 0.070ms > > --- > > For those curious, using the same printouts as I did above, this is what > it actually looks like on the destination side: > > * Destination VM is started * > > vhost_vdpa_set_owner() - register memory listener > vhost_vdpa_reset_device() - unregistering listener > > * Start live migration on source VM * > (qemu) migrate unix:/tmp/lm.sock > ... > > vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > vhost_vdpa_dev_start() - v->shared->listener_registered = 0, started = 1 > vhost_vdpa_dev_start() - register listener >
That's weird, can you check why the memory listener is not registered at vhost_vdpa_set_owner? Or, if it is registered, why is it not registered by the time vhost_vdpa_dev_start is called? This changes the downtime a lot, more than half of the time is spent on this. So it is worth fixing it before continuing. > And this is very different than the churning we saw in my previous email > that happens on the source / single guest VM with vhost-vDPA and its > startup path. > > --- > > Again, apologies on the confusion this caused. This was my fault for not > being more careful. > No worries!
