On Tue, Jul 30, 2024 at 10:57 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote: > > On 2024/07/30 11:04, Jason Wang wrote: > > On Tue, Jul 30, 2024 at 12:43 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx> > >>>>>>>>>>>>>> Signed-off-by: Andrew Melnychecnko <andrew@xxxxxxxxxx> > >>>>>>>>>>>>> > >>>>>>>>>>>>> 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. Such fallback is by design since the introduction of vhost. > We don't provide in-QEMU > fallback for other offloads. Yes but what I want to say is that eBPF RSS is different from those segmentation offloads. And technically, Qemu can do fallback for offloads (as RSC did). Thanks > > Regards, > Akihiko Odaki >
