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

On Mon, Aug 05, 2024 at 04:27:43PM +0900, Akihiko Odaki wrote: > On 2024/08/04 22:08, Peter Xu wrote: >> On Sun, Aug 04, 2024 at 03:49:45PM +0900, Akihiko Odaki wrote: >>> On 2024/08/03 1:26, Peter Xu wrote: >>>> On Sat, Aug 03, 2024 at 12:54:51AM +0900, Akihiko Odaki wrote: >>>>>>>> I'm not sure if I read it right. Perhaps you meant something more generic >>>>>>>> than -platform but similar? >>>>>>>> >>>>>>>> For example, "-profile [PROFILE]" qemu cmdline, where PROFILE can be either >>>>>>>> "perf" or "compat", while by default to "compat"? >>>>>>> >>>>>>> "perf" would cover 4) and "compat" will cover 1). However neither of them >>>>>>> will cover 2) because an enum is not enough to know about all hosts. I >>>>>>> presented a design that will cover 2) in: >>>>>>> https://lore.kernel.org/r/2da4ebcd-2058-49c3-a4ec-8e60536e5cbb@daynix.com >>>>>> >>>>>> "-merge-platform" shouldn't be a QEMU parameter, but should be something >>>>>> separate. >>>>> >>>>> Do you mean merging platform dumps should be done with another command? I >>>>> think we will want to know the QOM tree is in use when implementing >>>>> -merge-platform. For example, you cannot define a "platform" when e.g., you >>>>> don't know what netdev backend (e.g., user, vhost-net, vhost-vdpa) is >>>>> connected to virtio-net devices. Of course we can include those information >>>>> in dumps, but we don't do so for VMState. >>>> >>>> What I was thinking is the generated platform dump shouldn't care about >>>> what is used as backend: it should try to probe whatever is specified in >>>> the qemu cmdline, and it's the user's job to make sure the exact same qemu >>>> cmdline is used in other hosts to dump this information. >>>> >>>> IOW, the dump will only contain the information that was based on the qemu >>>> cmdline. E.g., if it doesn't include virtio device at all, and if we only >>>> support such dump for virtio, it should dump nothing. >>>> >>>> Then the -merge-platform will expect all dumps to look the same too, >>>> merging them with AND on each field. >>> >>> I think we will still need the QOM tree in that case. I think the platform >>> information will look somewhat similar to VMState, which requires the QOM >>> tree to interpret. >> >> Ah yes, I assume you meant when multiple devices can report different thing >> even if with the same frontend / device type. QOM should work, or anything >> that can identify a device, e.g. with id / instance_id attached along with >> the device class. >> >> One thing that I still don't know how it works is how it interacts with new >> hosts being added. >> >> This idea is based on the fact that the cluster is known before starting >> any VM. However in reality I think it can happen when VMs started with a >> small cluster but then cluster extended, when the -merge-platform has been >> done on the smaller set. >> >>> >>>> >>>> Said that, I actually am still not clear on how / whether it should work at >>>> last. At least my previous concern (1) didn't has a good answer yet, on >>>> what we do when profile collisions with qemu cmdlines. So far I actually >>>> still think it more straightforward that in migration we handshake on these >>>> capabilities if possible. >>>> >>>> And that's why I was thinking (where I totally agree with you on this) that >>>> whether we should settle a short term plan first to be on the safe side >>>> that we start with migration always being compatible, then we figure the >>>> other approach. That seems easier to me, and it's also a matter of whether >>>> we want to do something for 9.1, or leaving that for 9.2 for USO*. >>> >>> I suggest disabling all offload features of virtio-net with 9.2. >>> >>> I want to keep things consistent so I want to disable all at once. This >>> change will be very uncomfortable for us, who are implementing offload >>> features, but I hope it will motivate us to implement a proper solution. >>> >>> That said, it will be surely a breaking change so we should wait for 9.1 >>> before making such a change. >> >> Personally I don't worry too much on other offload bits besides USO* so far >> if we have them ON for longer time. My wish was that they're old good >> kernel features mostly supported everywhere who runs QEMU, then we're good. > > Unfortunately, we cannot expect everyone runs Linux, and the offload > features are provided by Linux. However, QEMU can run on other platforms, > and offload features may be provided by vhost-user or vhost-vdpa. I see. I am not familiar with the status quo there, so I'll leave that to you and other experts that know better on this.. Personally I do care more on Linux, as that's what we ship within RH.. > >> >> And I definitely worry about future offload features, or any feature that >> may probe host like this and auto-OFF: I hope we can do them on the safe >> side starting from day1. >> >> So I don't know whether we should do that to USO* only or all. But I agree >> with you that'll definitely be cleaner. >> >> On the details of how to turn them off properly.. Taking an example if we >> want to turn off all the offload features by default (or simply we replace >> that with USO-only).. >> >> Upstream machine type is flexible to all kinds of kernels, so we may not >> want to regress anyone using an existing machine type even on perf, >> especially if we want to turn off all. >> >> In that case we may need one more knob (I'm assuming this is virtio-net >> specific issue, but maybe not; using it as an example) to make sure the old >> machine types perfs as well, with: >> >> - x-virtio-net-offload-enforce >> >> When set, the offload features with value ON are enforced, so when >> the host doesn't support a offload feature it will fail to boot, >> showing the error that specific offload feature is not supported by the >> virtio backend. >> >> When clear, the offload features with value ON are not enforced, so >> these features can be automatically turned OFF when it's detected the >> backend doesn't support them. This may bring best perf but has the >> risk of breaking migration. > > "[PATCH v3 0/5] virtio-net: Convert feature properties to OnOffAuto" adds > "x-force-features-auto" compatibility property to virtio-net for this > purpose: > https://lore.kernel.org/r/20240714-auto-v3-0-e27401aabab3@daynix.com Ah ok. But note that there's still a slight difference: we need to avoid AUTO being an option, at all, IMHO. It's about making qemu cmdline the ABI: when with AUTO it's still possible the user uses AUTO on both sides, then ABI may not be guaranteed. AUTO would be fine if: (1) the property doesn't affect guest ABI, or (2) the AUTO bit will always generate the same thing on both hosts. However USO* isn't such case.. so the AUTO option is IMHO not wanted. What I mentioned above "x-virtio-net-offload-enforce" shouldn't add anything new to "uso"; it still can only be ON/OFF. However it should affect "flip that to OFF automatically" or "fail the boot" behavior on missing features.