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

[Alexei Starovoitov]

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.