Re: [PATCH v2 4/4] virtio-net: Add support for USO features

On Tue, Jul 30, 2024 at 12:43 AM Akihiko Odaki <akihiko.odaki(a)daynix.com&gt; wrote: > > On 2024/07/29 23:29, Peter Xu wrote: >> On Mon, Jul 29, 2024 at 01:45:12PM +0900, Akihiko Odaki wrote: >>> On 2024/07/29 12:50, Jason Wang wrote: >>>> On Sun, Jul 28, 2024 at 11:19 PM Akihiko Odaki <akihiko.odaki(a)daynix.com&gt; wrote: >>>>> >>>>> On 2024/07/27 5:47, Peter Xu wrote: >>>>>> On Fri, Jul 26, 2024 at 04:17:12PM +0100, Daniel P. Berrangé wrote: >>>>>>> On Fri, Jul 26, 2024 at 10:43:42AM -0400, Peter Xu wrote: >>>>>>>> On Fri, Jul 26, 2024 at 09:48:02AM +0100, Daniel P. Berrangé wrote: >>>>>>>>> On Fri, Jul 26, 2024 at 09:03:24AM +0200, Thomas Huth wrote: >>>>>>>>>> On 26/07/2024 08.08, Michael S. Tsirkin wrote: >>>>>>>>>>> On Thu, Jul 25, 2024 at 06:18:20PM -0400, Peter Xu wrote: >>>>>>>>>>>> On Tue, Aug 01, 2023 at 01:31:48AM +0300, Yuri Benditovich wrote: >>>>>>>>>>>>> USO features of virtio-net device depend on kernel ability >>>>>>>>>>>>> to support them, for backward compatibility by default the >>>>>>>>>>>>> features are disabled on 8.0 and earlier. >>>>>>>>>>>>> >>>>>>>>>>>>> Signed-off-by: Yuri Benditovich <yuri.benditovich(a)daynix.com&gt; >>>>>>>>>>>>> Signed-off-by: Andrew Melnychecnko <andrew(a)daynix.com&gt; >>>>>>>>>>>> >>>>>>>>>>>> Looks like this patch broke migration when the VM starts on a host that has >>>>>>>>>>>> USO supported, to another host that doesn't.. >>>>>>>>>>> >>>>>>>>>>> This was always the case with all offloads. The answer at the moment is, >>>>>>>>>>> don't do this. >>>>>>>>>> >>>>>>>>>> May I ask for my understanding: >>>>>>>>>> "don't do this" = don't automatically enable/disable virtio features in QEMU >>>>>>>>>> depending on host kernel features, or "don't do this" = don't try to migrate >>>>>>>>>> between machines that have different host kernel features? >>>>>>>>>> >>>>>>>>>>> Long term, we need to start exposing management APIs >>>>>>>>>>> to discover this, and management has to disable unsupported features. >>>>>>>>>> >>>>>>>>>> Ack, this likely needs some treatments from the libvirt side, too. >>>>>>>>> >>>>>>>>> When QEMU automatically toggles machine type featuers based on host >>>>>>>>> kernel, relying on libvirt to then disable them again is impractical, >>>>>>>>> as we cannot assume that the libvirt people are using knows about >>>>>>>>> newly introduced features. Even if libvirt is updated to know about >>>>>>>>> it, people can easily be using a previous libvirt release. >>>>>>>>> >>>>>>>>> QEMU itself needs to make the machine types do that they are there >>>>>>>>> todo, which is to define a stable machine ABI. >>>>>>>>> >>>>>>>>> What QEMU is missing here is a "platform ABI" concept, to encode >>>>>>>>> sets of features which are tied to specific platform generations. >>>>>>>>> As long as we don't have that we'll keep having these broken >>>>>>>>> migration problems from machine types dynamically changing instead >>>>>>>>> of providing a stable guest ABI. >>>>>>>> >>>>>>>> Any more elaboration on this idea? Would it be easily feasible in >>>>>>>> implementation? >>>>>>> >>>>>>> In terms of launching QEMU I'd imagine: >>>>>>> >>>>>>> $QEMU -machine pc-q35-9.1 -platform linux-6.9 ...args... >>>>>>> >>>>>>> Any virtual machine HW features which are tied to host kernel features >>>>>>> would have their defaults set based on the requested -platform. The >>>>>>> -machine will be fully invariant wrt the host kernel. >>>>>>> >>>>>>> You would have -platform hlep to list available platforms, and >>>>>>> corresonding QMP "query-platforms" command to list what platforms >>>>>>> are supported on a given host OS. >>>>>>> >>>>>>> Downstream distros can provide their own platforms definitions >>>>>>> (eg "linux-rhel-9.5") if they have kernels whose feature set >>>>>>> diverges from upstream due to backports. >>>>>>> >>>>>>> Mgmt apps won't need to be taught about every single little QEMU >>>>>>> setting whose default is derived from the kernel. Individual >>>>>>> defaults are opaque and controlled by the requested platform. >>>>>>> >>>>>>> Live migration has clearly defined semantics, and mgmt app can >>>>>>> use query-platforms to validate two hosts are compatible. >>>>>>> >>>>>>> Omitting -platform should pick the very latest platform that is >>>>>>> cmpatible with the current host (not neccessarily the latest >>>>>>> platform built-in to QEMU). >>>>>> >>>>>> This seems to add one more layer to maintain, and so far I don't know >>>>>> whether it's a must. >>>>>> >>>>>> To put it simple, can we simply rely on qemu cmdline as "the guest ABI"? I >>>>>> thought it was mostly the case already, except some extremely rare >>>>>> outliers. >>>>>> >>>>>> When we have one host that boots up a VM using: >>>>>> >>>>>> $QEMU1 $cmdline >>>>>> >>>>>> Then another host boots up: >>>>>> >>>>>> $QEMU2 $cmdline -incoming XXX >>>>>> >>>>>> Then migration should succeed if $cmdline is exactly the same, and the VM >>>>>> can boot up all fine without errors on both sides. >>>>>> >>>>>> AFAICT this has nothing to do with what kernel is underneath, even not >>>>>> Linux? I think either QEMU1 / QEMU2 has the option to fail. But if it >>>>>> didn't, I thought the ABI should be guaranteed. >>>>>> >>>>>> That's why I think this is a migration violation, as 99.99% of other device >>>>>> properties should be following this rule. The issue here is, we have the >>>>>> same virtio-net-pci cmdline on both sides in this case, but the ABI got >>>>>> break. >>>>>> >>>>>> That's also why I was suggesting if the property contributes to the guest >>>>>> ABI, then AFAIU QEMU needs to: >>>>>> >>>>>> - Firstly, never quietly flipping any bit that affects the ABI... >>>>>> >>>>>> - Have a default value of off, then QEMU will always allow the VM to boot >>>>>> by default, while advanced users can opt-in on new features. We can't >>>>>> make this ON by default otherwise some VMs can already fail to boot, >>>>> >>>>> It may not be necessary the case that old features are supported by >>>>> every systems. In an extreme case, a user may migrate a VM from Linux to >>>>> Windows, which probably doesn't support any offloading at all. A more >>>>> convincing scenario is RSS offloading with eBPF; using eBPF requires a >>>>> privilege so we cannot assume it is always available even on the latest >>>>> version of Linux. >>>> >>>> I don't get why eBPF matters here. It is something that is not noticed >>>> by the guest and we have a fallback anyhow. > > It is noticeable for the guest, and the fallback is not effective with > vhost. It's a bug then. Qemu can fallback to tuntap if it sees issues in vhost.