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

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).