On Mon, Aug 30, 2021 at 9:10 AM Markus Armbruster <arm...@redhat.com> wrote: > > Yuri Benditovich <yuri.benditov...@daynix.com> writes: > > > On Tue, Aug 24, 2021 at 9:41 AM Markus Armbruster <arm...@redhat.com> wrote: > >> > >> Andrew Melnichenko <and...@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. > >> >
