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

[Yuri Benditovich]

On Mon, Aug 30, 2021 at 9:10 AM Markus Armbruster <armbru@redhat.com> wrote:
>
> Yuri Benditovich <yuri.benditovich@daynix.com> writes:
>
> > 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?
>
> I'll explain in more detail, but first I'd like you to answer my
> question below.
>
> >> 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.
>
> You did not actually answer my question :)
>
> So let's try again: if libvirt does use the wrong RSS helper, how does
> the system behave?

The receive-side scaling may work incorrectly, i.e. finally may move
incoming packets to a virtqueue different than expected 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.
> >>
>