On Fri, Jan 9, 2026 at 4:08 AM Morten Brørup <[email protected]> wrote: > > +Aaron, please read up on this discussion, and step in if you can help. > > (Aaron is the DPDK Project testing leader, and works at Red Hat.) > > > From: Scott Mitchell [mailto:[email protected]] > > Sent: Friday, 9 January 2026 05.58 > > > > On Thu, Jan 8, 2026 at 7:01 PM Stephen Hemminger > > <[email protected]> wrote: > > > > > > On Thu, 8 Jan 2026 16:19:37 -0500 > > > Scott Mitchell <[email protected]> wrote: > > > > > > > On Thu, Jan 8, 2026 at 11:12 AM Stephen Hemminger > > > > <[email protected]> wrote: > > > > > > > > > > On Thu, 8 Jan 2026 01:13:38 -0500 > > > > > [email protected] wrote: > > > > > > > > > > > +#ifdef RTE_CC_GCC > > > > > > + /* Suppress GCC -Wmaybe-uninitialized false positive. No > > assembly/runtime impacts. */ > > > > > > + asm volatile("" : "+m" (psd_hdr)); > > > > > > +#endif > > > > > > > > > > > > > > > > Maybe rte_compiler_barrier() will do same thing? > > > > > > > > Agreed it feels like a compiler bug but looking for advice if I'm > > > > missing something :) > > > > > > > > My initial concern with rte_compiler_barrier is its a general > > barrier > > > > which may have broader impacts on > > > > optimizations and compiled code. Will that be an issue in this > > case? I > > > > wasn't sure and the approach > > > > in the patch is targeted at a specific variable and assembly from > > > > clang/gcc was the same. I will > > > > introduce a macro to make it cleaner and I can replace it with > > > > rte_compiler_barrier if preferred. > > > > > > Maybe try with -fanalyzer and it might tell you more. > > > I suspect some of the aliasing setting are causing issues. > > > Some drivers are turning on no-strict-aliasing > > > > I have more evidence this is a GCC optimizer bug. > > The RTE_SUPPRESS_UNINITIALIZED_WARNING approach serves > > as a workaround to avoid the bug. I created a more minimal reproducer: > > https://gist.github.com/Scottmitch/bf23748b4588e68c9bdb8d124f92f1bd > > > > Your suspicion was correct, -fno-strict-aliasing avoids the bug but I > > don't > > think it is desirable to enable this broadly for DPDK when we have a > > more targeted workaround. > > > > I will reach out to RH to confirm but in the interim I suggest we keep > > RTE_SUPPRESS_UNINITIALIZED_WARNING (or similar alternative). > > If this is a GCC compiler bug limited to the GCC version offered by RHEL 11, > I prefer splitting the patch into a series with the following steps: > Patch 1/2: Add the optimization and new test cases in their minimal form, > designed to work on normal compilers. Disregard bugs/warnings from the weird > RHEL 11 compiler. > I.e. don't modify lib/eal/include/rte_common.h, lib/net/rte_ip6.h, > lib/net/rte_ip4.h, drivers/net/hinic/hinic_pmd_tx.c, > drivers/net/mlx5/mlx5_flow_dv.c. > Patch 2/2: Add the workarounds required by the RHEL 11 compiler. > > Also, the change to drivers/net/hinic/hinic_pmd_tx.c should be moved to a > patch independent of this series. > It's not directly related to this series, so let's not add more to the > discussion than we need to. ;-) > And the implementation in the driver only considers RTE_MBUF_F_TX_TCP_SEG, > whereas the DPDK function also considers RTE_MBUF_F_TX_UDP_SEG, so it > warrants a separate discussion; it possibly fixes a bug. > > Maybe even move the RHEL 11 related patches (my suggested patch 2/2) into a > separate series, for the same conceptual reasons as moving the HINIC driver > patch into a separate series. > You can use the Depends-On tag > (https://doc.dpdk.org/guides/contributing/patches.html#patch-dependencies) > for the follow-on changes to __rte_raw_cksum(). >
General strategy makes sense! The GCC bug showed up on dpdk CI on non-RH platforms too https://github.com/ovsrobot/dpdk/actions/runs/20744990187/job/59560072349. There is also a ubsan warning flagged while running the new fuzz test (https://github.com/ovsrobot/dpdk/actions/runs/20821548318/job/59811205423). Seems like a false positive and I added __rte_no_ubsan_alignment to get a clean build. I'll restructure the changes following this approach: Series: "net: optimize raw checksum computation" (3 patches) 1/3: net: optimize __rte_raw_cksum and add tests 2/3: add workaround for UBSAN alignment false positive 3/3: add workaround for GCC optimization bug I'll defer the hinic refactoring (switching to common checksum functions) to a separate follow-up series as suggested, since it's independent of the core
