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://lore.kernel.org/qemu-devel/6c8ebb97-d546-3f1c-4cdd-54e23a566...@nvidia.com/T/ 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. > If it's only about pre-heat, could dest qemu preheat with max num of > vqueues? Is it the same cost of downtime when growing num of queues, > v.s. shrinking num of queues? > Well you need to send the vq addresses and properties to preheat these. If the address is invalid, the destination device will interpret the vq address as the avail ring, for example, and will read an invalid avail idx. > For softwares, is it about memory transaction updates due to the vqueues? > If so, have we investigated a more generic approach on memory side, likely > some form of continuation from Chuang's work I previously mentioned? > This work is very interesting, and most of the downtime was because of memory pinning indeed. Thanks for bringing it up! But the downtime is not caused for the individual vq memory config, but for pinning all the guest's memory for the device to access to it. I think it is worth exploring if it affects the downtime in the case of HW. I don't see any reason to reject that series but lack of reviews, isn't it?
