Re: [PATCH 5/5] qmp: Added qemu-ebpf-rss-path command.

[Andrew Melnichenko]

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.