Re: [PATCH v2 bpf-next 00/13] Atomics for eBPF

[Yonghong Song]

On 11/30/20 9:22 AM, Yonghong Song wrote:
> On 11/28/20 5:40 PM, Alexei Starovoitov wrote:
> > 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.
>
> We can initially all have BPF_FETCH bit as at that point we do not
> have def-use chain. Later on we can add a
> machine ssa IR phase and check whether the result of, say 
> atomic_fetch_or(), is used or not. If not, we can change the
> instruction to atomic_or.

Just implemented what we discussed above in llvm:
  https://reviews.llvm.org/D72184
main change:
  1. atomic_fetch_sub (and later atomic_sub) is gone. llvm will
     transparently transforms it to negation followed by
     atomic_fetch_add or atomic_add (xadd). Kernel can remove
     atomic_fetch_sub/atomic_sub insns.
  2. added new instructions for atomic_{and, or, xor}.
  3. for gcc builtin e.g., __sync_fetch_and_or(), if return
     value is used, atomic_fetch_or will be generated. Otherwise,
     atomic_or will be generated.