On 2024/07/30 12:03, Jason Wang wrote:
On Tue, Jul 30, 2024 at 10:57 AM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
On 2024/07/30 11:04, Jason Wang wrote:
On Tue, Jul 30, 2024 at 12:43 AM Akihiko Odaki <akihiko.odaki@daynix.com> 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@daynix.com> 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@daynix.com> >>>>>>>>>> Signed-off-by: Andrew Melnychecnko <andrew@daynix.com> >>>>>>>>> >>>>>>>>> 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