Mark Charney OS <mark.char...@os.amperecomputing.com> writes: > Using QEMU v8.2.0 (and also the HEAD of the git master branch), I > encountered an unexpected situation: an ldff1b is returning 1 byte > when I run with the QEMU user level plugin (and setting FFR as if > there was a fault). > > However the ldff1b actually loads 16 bytes when: (a) I run this same > test natively on a system with SVE support (no QEMU involved) or (b) > when I run this test interactively (logged in to a console) in GDB > running on the QEMU (with no plugin involved). > > I was wondering if this one-byte-per-ldff1b was a known/expected > behavior with plugins? I guess it is legal to only return one byte, > but I was wondering why QEMU did this and if there was some way to get > QEMU to return 16 bytes in the absence of faults (or as many bytes as > it can up until the fault).
Could it be a change with the location w.r.t a page boundary between the two cases? > > There is *no* page boundary being crossed in the examples of interest, > and no MMIO, so a partial data return is not expected. The page > referenced is mapped and previously referenced. > > Talking to Alex Bennee, he pointed out: > >> I'm wondering if this is a result of the fix in 6d03226b422 >> (plugins: force slow path when plugins instrument memory ops). This >> will always force the slow path which is where we instrument the >> operation. > > I attempted to revert this commit locally and no longer got memop > callbacks for any SVE load operations, first fault, nonfault or not > "normal" predicated SVE operations. But I believe ldff1b are returning > 16 bytes (judging by the control flow). > > Our goal is to use QEMU for tracing with a home-grown plugin. For our > purposes, we were expecting to observe control flow like what we see > on SVE-enabled hardware where ldff1b returns 16 bytes in the absence > of faults. > > If necessary, I can provide a reproducer, that includes: > - a sve strcpy loop from one of Alex's talks. Yeah lets add the test case. > - a simple user level plugin It should show up with execlog as well right? -- Alex Bennée Virtualisation Tech Lead @ Linaro
