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. Enabling RSS by default will result in a similar problem although it is older than USO.
- 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.
The point is libvirt can explicitly set "on" to avoid the "auto" behavior. libvirt does not have to use the "auto" value.
libvirt can still use "auto" if desired. virDomainNetDefParseXMLDriver() in libvirt actually parses tristate values (libvirt uses "default" instead of "auto" as the mnemonic) for these features though "default" is currently disabled by the schema (src/conf/schemas/domaincommon.rng). Allowing user to specify "default" is only a matter of editing the schema. Of course specifying "default" will make the VM unsafe for migration.
Isn't keeping the default AUTO the same as before when it used to be ON? I mean, AUTO in a qemu cmdline doesn't guarantee guest API either.
True. It only deals with the situation that "the host doesn't support the feature while the cmdline enabled it".
Indeed it looks like it's a step forward to make ON having the clear semantics of "fail when unsupported". It's just that I am not sure how useful is AUTO here, because anyway we'll need to break ON semantics even with AUTO, so that an old QEMU script with USO=ON used to boot on old kernels but not it won't. > What I was trying to say is whether we should make the default parameter to be migratable. IOW, it looks to me AUTO should deserve a migration blocker when chosen.
After all, Libvirt hopefully shouldn't use AUTO at all but only ON/OFF, while any user when not caring much on these perf details should always use OFF on any kernel dependent features that may affect the guest ABI.
Well, there should be libvirt users who care performance and do not use migration so it's better for them if libvirt can use auto. But the use of "auto" should be mutually exclusive with migration of course. Regards, Akihiko Odaki