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

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. > >> >> - If the host doesn't support the feature while the cmdline enabled it, >> it needs to fail QEMU boot rather than flipping, so that it says "hey, >> this host does not support running such VM specified, due to XXX >> feature missing". > > This is handled in: > > "virtio-net: Convert feature properties to OnOffAuto" > https://patchew.org/QEMU/20240714-auto-v3-0-e27401aabab3@daynix.com/ I may miss something but I think "Auto" doesn't make sense to libvirt.