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

On 2024/07/30 11:04, Jason Wang wrote: > 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. We can certainly fallback to in-QEMU RSS by disabling vhost, but I would not say lack of such fallback is a bug.

We don't provide in-QEMU fallback for other offloads.

Regards, Akihiko Odaki