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

On 2024/07/30 12:03, Jason Wang wrote: > On Tue, Jul 30, 2024 at 10:57 AM Akihiko Odaki <akihiko.odaki(a)daynix.com&gt; wrote: >> >> 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. > > 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). Well, I couldn't find any code disabling vhost for the in-QEMU RSC implementation.

Looking at the code, I also found the case of vhost-vdpa. vhost can be simply disabled if it is backed by tuntap, but it is not the case for vDPA.

Regards, Akihiko Odaki