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client-ip=205.220.165.32; envelope-from=si-wei.liu@oracle.com; helo=mx0a-00069f02.pphosted.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Hi Jonah, On 8/15/2025 7:50 AM, Jonah Palmer wrote: > > > On 8/14/25 5:28 AM, Eugenio Perez Martin wrote: >> On Wed, Aug 13, 2025 at 4:06 PM Peter Xu 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 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-54e23a566f61@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: I guess what Eugenio may want to see is the config with SVQ=off (i.e. without x-svq=on in below netdev line). Do you have number for that as well? Then since vhost_vdpa_dev_start() it should exclude the time for pinning, you could easily profile/measure vq configure time (the CVQ commands to configure vq number, size, RSS, etc) vs dataplane enablement, same way as you did for SVQ=on. Regards, -Siwei > > -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 > >>> 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? >> >