On Fri, Nov 27, 2020 at 09:53:05PM -0800, Yonghong Song wrote: > > > On 11/27/20 9:57 AM, Brendan Jackman wrote: > > Status of the patches > > ===================== > > > > Thanks for the reviews! Differences from v1->v2 [1]: > > > > * Fixed mistakes in the netronome driver > > > > * Addd sub, add, or, xor operations > > > > * The above led to some refactors to keep things readable. (Maybe I > > should have just waited until I'd implemented these before starting > > the review...) > > > > * Replaced BPF_[CMP]SET | BPF_FETCH with just BPF_[CMP]XCHG, which > > include the BPF_FETCH flag > > > > * Added a bit of documentation. Suggestions welcome for more places > > to dump this info... > > > > The prog_test that's added depends on Clang/LLVM features added by > > Yonghong in https://reviews.llvm.org/D72184 > > > > This only includes a JIT implementation for x86_64 - I don't plan to > > implement JIT support myself for other architectures. > > > > Operations > > ========== > > > > This patchset adds atomic operations to the eBPF instruction set. The > > use-case that motivated this work was a trivial and efficient way to > > generate globally-unique cookies in BPF progs, but I think it's > > obvious that these features are pretty widely applicable. The > > instructions that are added here can be summarised with this list of > > kernel operations: > > > > * atomic[64]_[fetch_]add > > * atomic[64]_[fetch_]sub > > * atomic[64]_[fetch_]and > > * atomic[64]_[fetch_]or > > * atomic[64]_[fetch_]xor > > > * atomic[64]_xchg > > * atomic[64]_cmpxchg > > Thanks. Overall looks good to me but I did not check carefully > on jit part as I am not an expert in x64 assembly... > > This patch also introduced atomic[64]_{sub,and,or,xor}, similar to > xadd. I am not sure whether it is necessary. For one thing, > users can just use atomic[64]_fetch_{sub,and,or,xor} to ignore > return value and they will achieve the same result, right? > From llvm side, there is no ready-to-use gcc builtin matching > atomic[64]_{sub,and,or,xor} which does not have return values. > If we go this route, we will need to invent additional bpf > specific builtins. I think bpf specific builtins are overkill. As you said the users can use atomic_fetch_xor() and ignore return value. I think llvm backend should be smart enough to use BPF_ATOMIC | BPF_XOR insn without BPF_FETCH bit in such case. But if it's too cumbersome to do at the moment we skip this optimization for now.
