On 2024/08/03 1:26, Peter Xu wrote:
On Sat, Aug 03, 2024 at 12:54:51AM +0900, Akihiko Odaki wrote:
I'm not sure if I read it right. Perhaps you meant something more generic
than -platform but similar?
For example, "-profile [PROFILE]" qemu cmdline, where PROFILE can be either
"perf" or "compat", while by default to "compat"?
"perf" would cover 4) and "compat" will cover 1). However neither of them
will cover 2) because an enum is not enough to know about all hosts. I
presented a design that will cover 2) in:
https://lore.kernel.org/r/2da4ebcd-2058-49c3-a4ec-8e60536e5cbb@xxxxxxxxxx
"-merge-platform" shouldn't be a QEMU parameter, but should be something
separate.
Do you mean merging platform dumps should be done with another command? I
think we will want to know the QOM tree is in use when implementing
-merge-platform. For example, you cannot define a "platform" when e.g., you
don't know what netdev backend (e.g., user, vhost-net, vhost-vdpa) is
connected to virtio-net devices. Of course we can include those information
in dumps, but we don't do so for VMState.
What I was thinking is the generated platform dump shouldn't care about
what is used as backend: it should try to probe whatever is specified in
the qemu cmdline, and it's the user's job to make sure the exact same qemu
cmdline is used in other hosts to dump this information.
IOW, the dump will only contain the information that was based on the qemu
cmdline. E.g., if it doesn't include virtio device at all, and if we only
support such dump for virtio, it should dump nothing.
Then the -merge-platform will expect all dumps to look the same too,
merging them with AND on each field.
I think we will still need the QOM tree in that case. I think the
platform information will look somewhat similar to VMState, which
requires the QOM tree to interpret.
Said that, I actually am still not clear on how / whether it should work at
last. At least my previous concern (1) didn't has a good answer yet, on
what we do when profile collisions with qemu cmdlines. So far I actually
still think it more straightforward that in migration we handshake on these
capabilities if possible.
And that's why I was thinking (where I totally agree with you on this) that
whether we should settle a short term plan first to be on the safe side
that we start with migration always being compatible, then we figure the
other approach. That seems easier to me, and it's also a matter of whether
we want to do something for 9.1, or leaving that for 9.2 for USO*.
I suggest disabling all offload features of virtio-net with 9.2.
I want to keep things consistent so I want to disable all at once. This
change will be very uncomfortable for us, who are implementing offload
features, but I hope it will motivate us to implement a proper solution.
That said, it will be surely a breaking change so we should wait for 9.1
before making such a change.
By the way, I am wondering perhaps the "no-cross-migrate" scenario can
be implemented relatively easy in a way similar to compatibility
properties. The idea is to add the "no-cross-migrate" property to
machines. If the property is set to "on", all offload features of
virtio-net will be set to "auto". virtio-net will then probe the offload
features and enable available offloading features.
Regards,
Akihiko Odaki
