On 29.07.26 14:30, Markus Armbruster wrote:
Vladimir Sementsov-Ogievskiy <[email protected]> writes:
Support transferring of TAP state (including open fd).
Add new property "x-local-migration-supported", which defines
whether local-migration is actually supported for this TAP device.
Starting from 11.2 QEMU Machine Types it's enabled by default.
Note that local-migration is enabled by global "local" migration
parameter, but individual devices may have additional options to
enable/disable it per device.
The tricky thing is that we need to know whether to call open/connect in
TAP initialization code, i.e. we need to know the value of migration
parameter "local" when creating the TAP device. For incoming migration,
we can know only for TAP devices created with QMP after setting the
migration parameter with QMP.
So the full picture is:
On source, to start outgoing "local" migration you need:
- migration parameter "local" set to true
- "x-local-migration-supported" TAP option set to true (the
default, starting from 11.2 QEMU Machine Types)
Is the machine type part still accurate? The description in the QAPI
schema has (default: false, since 11.2).
Oops, need to update commit message.
If at least one of these options is not set, TAP backend
doesn't participate in migration.
On target, things are more difficult:
Same, you need both "local" and "x-local-migration-supported"
be set. And same, if one of them is not set, TAP backend
is initialized as usual, and doesn't accept any incoming
state.
Additionally, if you are going to set "local", it must be
set before creating the TAP device. If TAP device created
with "local" unset, it initializes as usual. If you enable
"local" after it and start incoming migration, it will fail
in .pre_load handler of TAP backend.
Moreover, there are interface restrictions: if you create TAP
device when QEMU is in INCOMING state, and both "local"
and "x-local-migration-supported" set, most of TAP options are
not allowed, and script/downscript are required to be explicitly
unset (set to "" or "no").
Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Reviewed-by: Ben Chaney <[email protected]>
[...]
diff --git a/qapi/net.json b/qapi/net.json
index acb8594c952..e6921912e12 100644
--- a/qapi/net.json
+++ b/qapi/net.json
@@ -437,6 +437,29 @@
# @poll-us: maximum number of microseconds that could be spent on busy
# polling for tap (since 2.7)
#
+# @x-local-migration-supported: enable local migration for this TAP
+# backend. When set, local migration is enabled/disabled by
+# migration parameter @local for this TAP backend. When unset,
+# migration parameter @local is ignored for this TAP backend.
+# To be able to do incoming local migration of a TAP backend,
+# migration parameter @local must be set _before_ creating the
+# TAP backend. Otherwise, TAP backend is initialized as usual,
+# opening/creating TAP devices in kernel. In this case further
+# local incoming migration (with migration parameter @local set
+# after creating TAP backend with @x-local-migration-supporeted
+# parameter set) will simply fail.
Either have a a blank line here so you actually get two paragraphs, or
refill the entire description to avoid the illusion of two paragraphs.
+# Moreover, when QEMU is in incoming migration state, migration
+# parameter @local is set and @x-local-migration-supported is set,
+# the following options are not supported and must not be set:
+# @fd, @fds, @helper, @br, @ifname, @sndbuf, @vnet_hdr.
+# Additionally in this case @script and @downscipt must be
@downscript
+# explicitly disabled (empty strings or "no").
"no" is deprecated [PATCH 3]. We'll have to remember deleting 'or "no"'
here when remove it. Easy to forget. Delete it now?
Maybe
# Additionally, @script and @downscript must be explicitly disabled
# then.
Agree.
+# (default: false, since 11.2)
+#
+# Features:
+#
+# @unstable: Member @x-local-migration-supported is experimental.
+#
# Since: 1.2
##
{ 'struct': 'NetdevTapOptions',
@@ -455,7 +478,9 @@
'*vhostfds': 'str',
'*vhostforce': 'bool',
'*queues': 'uint32',
- '*poll-us': 'uint32'} }
+ '*poll-us': 'uint32',
+ '*x-local-migration-supported': {
+ 'type': 'bool', 'features' : [ 'unstable'] } } }
##
# @NetdevSocketOptions:
Naming is hard...
"Supported" sounds like a property of the QEMU process. That's not what
this is. It's an on/off switch that happens to be in series with
another on/off switch, namely migration parameter @local.
Maybe
@permit-local-migration: permit local migration for this TAP
backend. When set, local migration is enabled/disabled by
migration parameter @local for this TAP backend. ...
Good for me. Will use, if no more opinions.
Or x-permit-local-migration? I've added "x-" prefix as Michael suggested.
And it seems reasonable, as it's not only QAPI field, but also QOM property,
and we don't have "unstable" features for properties.
--
Best regards,
Vladimir
