Public bug reported:
[impact]
when using qemu with interfaces that use the vhost_user backend (i.e.
dpdk), there is a hardcoded limit of 8 memory regions that the
vhost_user backend driver can support. This hardcoded limit is still
present upstream, and appears to be a limitation of the vhost-user
specification's "set mem table" protocol.
There are some optimization and bugfixes in code over time, and in
particular there is a very recent upstream commit
(27598393a23215cfbf92ad550b9541675b0b8f2b) that allows increasing this
max, however the vhost-user backend (that qemu talks to) must support
and negotiate to use the new method.
[test case]
start a qemu guest with at least one vhost-user interface, and more than
8 discontiguous memory regions (which may result from multiple
passthrough pci devices or other causes). The vhost-user device will
fail to init due to exceeding its max memory region limit.
[regression potential]
tbd
[scope]
this limit still exists upstream, and appears to be a vhost-user
specification protocol limit, so the only resolution may to be to switch
to the new protocol feature added in the upstream commit referenced in
the impact section. thus, any change would likely be needed in all
releases, but as the new protocol feature is not a bug fix, but a new
feature, it's unlikely it would qualify for backporting to any/all SRU
releases.
however, see other info section.
[other info]
there are at least some, and possibly multiple, bugfixes through the
versions in x/b/f that improve how vhost ignores some memory regions,
and merges contiguous regions, so it's possible the limit of 8 could be
reached for some guests that fail on older releases.
tbd identifying any/all such patches that might help.
** Affects: qemu (Ubuntu)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1886704
Title:
qemu fails to init vhost_user if > 8 memory regions
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