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Description of problem:
The tx_queue_size is supported on libvirt libvirt-3.2.0-14.el7_4.3.x86_64, while 
set tx_queue_size to 1024 or 512 in interfaces other than vhostuser interface type makes no changes, the tx_queue size on guest will always keep as 256.

Suggested to document it somewhere like in /usr/share/doc/libvirt-docs-3.2.0/html/formatdomain.html#elementsDriverBackendOptions

Version-Release number of selected component (if applicable):
libvirt-docs-3.2.0-14.el7_4.3.x86_64

How reproducible:

Steps to Reproduce:
1.
2.
3.

Actual results:

Expected results:


Additional info:
Refer to https://bugzilla.redhat.com/show_bug.cgi?id=1482514#c14, "for vhost-user, we allow tx queue size to be greater than 256, for vhost-kernel, we don't allow more than 256." this is kernel limitation.

Patch proposed upstream:

https://www.redhat.com/archives/libvir-list/2017-August/msg00914.html

I've pushed the patch upstream:

commit b39464ba26424053ec5fd953c89087eb66d56d73
Author:     Michal Privoznik <mprivozn>
AuthorDate: Tue Aug 29 17:01:16 2017 +0200
Commit:     Michal Privoznik <mprivozn>
CommitDate: Wed Aug 30 08:49:48 2017 +0200

    docs: Document yet another limitation of tx_queue_size
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1484234
    
    Turns out, only vhostuser type of interfaces are supported
    currently.
    
    Signed-off-by: Michal Privoznik <mprivozn>

v3.7.0-rc1-4-gb39464ba2

checked on file:///usr/share/doc/libvirt-docs-3.7.0/html/formatdomain.html
It is documented:

"Moreover, some hypervisors may pose some restrictions on actual value. For instance, QEMU v2.9 requires value to be a power of two from [256, 1024] range. In addition to that, this may work only for a subset of interface types, e.g. aforementioned QEMU enables this option only for vhostuser type. Since 3.7.0 (QEMU and KVM only)"

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHEA-2018:0704