On Mon, Aug 30, 2021 at 11:14 AM Markus Armbruster <arm...@redhat.com> wrote: > > Yuri Benditovich <yuri.benditov...@daynix.com> writes: > > > 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. > > Then I'm confused about the purpose of "the stamp" mentioned below. Can > you enlighten me?
The stamp is a string (common for qemu executable and RSS helper executable during build) that qemu can later retrieve from the helper in run-time and ensure this helper is fully compatible with this build of qemu (in terms of eBPF operation). The helper is built with the same C headers (related to ebpf operation) as the qemu, the qemu is able to receive file descriptors created by the helper (of ebpf program and ebpf data structure's maps) from libvirt and deal with them as if it has created them. > > > > >> > >> >> >> > >> >> > 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. >