Hi,

> The helper may or may not be installed at the path compiled into QEMU.
>
Yes, so the helper will not be called - QEMU will try to initiate eBPF RSS
or use "in-qemu" RSS.

What happens when you use the wrong helper?
>
UB - in most cases, eBPF program will work with wrong configurations.
That's why the stamp was added.

query-helper-paths checks the stamp only for RSS helper.

On Sat, Aug 7, 2021 at 3:54 PM Markus Armbruster <armbru@redhat.com> wrote:

> Andrew Melnychenko <andrew@daynix.com> writes:
>
> > New qmp command to query ebpf helper.
> > It's crucial that qemu and helper are in sync and in touch.
> > Technically helper should pass eBPF fds that qemu may accept.
> > And different qemu's builds may have different eBPF programs and helpers.
> > Qemu returns helper that should "fit" to virtio-net.
> >
> > Signed-off-by: Andrew Melnychenko <andrew@daynix.com>
>
> [...]
>
> > diff --git a/qapi/misc.json b/qapi/misc.json
> > index 156f98203e..9aaf8fbcca 100644
> > --- a/qapi/misc.json
> > +++ b/qapi/misc.json
> > @@ -519,3 +519,36 @@
> >   'data': { '*option': 'str' },
> >   'returns': ['CommandLineOptionInfo'],
> >   'allow-preconfig': true }
> > +
> > +##
> > +# @HelperPath:
> > +#
> > +# Name of the helper and binary location.
> > +##
> > +{ 'struct': 'HelperPath',
> > +  'data': {'name': 'str', 'path': 'str'} }
> > +
> > +##
> > +# @query-helper-paths:
> > +#
> > +# Query helper paths. Initially, this command was added for
> > +# qemu-ebpf-rss-helper. The qemu would check "the stamp" and
> > +# returns proper helper.
> > +#
> > +# Returns: list of object that contains name and path for helper.
> > +#
> > +# Since: 6.1
> > +#
> > +# Example:
> > +#
> > +# -> { "execute": "query-helper-paths" }
> > +# <- { "return": [
> > +#        {
> > +#          "name": "qemu-ebpf-rss-helper",
> > +#          "path": "/usr/local/libexec/qemu-ebpf-rss-helper"
> > +#        }
> > +#      ]
> > +#    }
> > +#
> > +##
> > +{ 'command': 'query-helper-paths', 'returns': ['HelperPath'] }
>
>
> Hmm.
>
> I understand the desire to help management applications to use the right
> helper.  But I'm not sure this command is actually useful.  The helper
> may or may not be installed at the path compiled into QEMU.
>
> What happens when you use the wrong helper?
>
> Even if we conclude this is the right approach for this helper, we still
> need to review the other helpers to see which of them we should have
> query-helper-paths cover.
>
>