On 8/18/25 2:51 AM, Eugenio Perez Martin wrote:
On Fri, Aug 15, 2025 at 4:50 PM Jonah Palmer <jonah.pal...@oracle.com> wrote:
On 8/14/25 5:28 AM, Eugenio Perez Martin wrote:
On Wed, Aug 13, 2025 at 4:06 PM Peter Xu <pet...@redhat.com> 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 <pet...@redhat.com> 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/6c8ebb97-d546-3f1c-4cdd-54e23a566...@nvidia.com/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().
---
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
