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

[Yuri Benditovich]

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.

Hi Markus,
Indeed the intention of this command is to return the proper helper.
Especially in the case of RSS helper this is *reliable* advice and it
points to the helper that was built together with QEMU, i.e. with the
same headers.
This was discussed earlier, for example in
https://lists.nongnu.org/archive/html/qemu-devel/2021-06/msg02248.html

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

What is the proper solution for the problem from your point of view?

>
> 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?

This one is required to be *fully synchronized* with the existing build of QEMU.
Other helpers are probably less restrictive and do not have common
structures definitions with the QEMU, otherwise they would face the
same problem.

>
> >> What happens when you use the wrong helper?

Our intention is that libvirt should never use the wrong RSS helper.
But it does not have any ability to check which helper is compatible
with the QEMU.
QEMU can easily recognize the correct one.

> >>
> > 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.
>