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

[Andrew Melnichenko]

Hi,

Yes, the stamp check was added.

So the qemu emulator should return a suitable RSS BPF helper or nothing.

Each qemu emulator may have a different helper that suits it.

So, the idea is to ask for the helper from qemu.

On Tue, Aug 24, 2021 at 9:41 AM Markus Armbruster <armbru@redhat.com> wrote:

Andrew Melnichenko <andrew@daynix.com> writes:

> 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.

My point is: the proposed command's mission is to help the management
application run the right helper.  However, its advice is *unreliable*.
It may point to the wrong helper, or to nothing at all.  The right
helper may still exist elsewhere.

I suspect you're trying to address the problem at the wrong level.

Similar versioning issues exist with other helpers.  We've been doing
fine without QEMU providing unreliable advice on where they might sit in
the file system.  What makes this one different?

>> 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.

I have no idea what you're talking about :)

My best guess is that you're trying to tell me that attempting to work
with the wrong helper will fail cleanly due to some stamp check.  That
would be nice.