Messages in this thread | | | Date | Wed, 11 Nov 2015 13:21:04 +0100 | From | Daniel Borkmann <> | Subject | Re: [PATCH 2/2] arm64: bpf: add BPF XADD instruction |
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On 11/11/2015 12:58 PM, Will Deacon wrote: > On Wed, Nov 11, 2015 at 11:42:11AM +0100, Daniel Borkmann wrote: >> On 11/11/2015 11:24 AM, Will Deacon wrote: >>> On Wed, Nov 11, 2015 at 09:49:48AM +0100, Arnd Bergmann wrote: >>>> On Tuesday 10 November 2015 18:52:45 Z Lim wrote: >>>>> On Tue, Nov 10, 2015 at 4:42 PM, Alexei Starovoitov >>>>> <alexei.starovoitov@gmail.com> wrote: >>>>>> On Tue, Nov 10, 2015 at 04:26:02PM -0800, Shi, Yang wrote: >>>>>>> On 11/10/2015 4:08 PM, Eric Dumazet wrote: >>>>>>>> On Tue, 2015-11-10 at 14:41 -0800, Yang Shi wrote: >>>>>>>>> aarch64 doesn't have native support for XADD instruction, implement it by >>>>>>>>> the below instruction sequence: >>>>> >>>>> aarch64 supports atomic add in ARMv8.1. >>>>> For ARMv8(.0), please consider using LDXR/STXR sequence. >>>> >>>> Is it worth optimizing for the 8.1 case? It would add a bit of complexity >>>> to make the code depend on the CPU feature, but it's certainly doable. >>> >>> What's the atomicity required for? Put another way, what are we racing >>> with (I thought bpf was single-threaded)? Do we need to worry about >>> memory barriers? >>> >>> Apologies if these are stupid questions, but all I could find was >>> samples/bpf/sock_example.c and it didn't help much :( >> >> The equivalent code more readable in restricted C syntax (that can be >> compiled by llvm) can be found in samples/bpf/sockex1_kern.c. So the >> built-in __sync_fetch_and_add() will be translated into a BPF_XADD >> insn variant. > > Yikes, so the memory-model for BPF is based around the deprecated GCC > __sync builtins, that inherit their semantics from ia64? Any reason not > to use the C11-compatible __atomic builtins[1] as a base?
Hmm, gcc doesn't have an eBPF compiler backend, so this won't work on gcc at all. The eBPF backend in LLVM recognizes the __sync_fetch_and_add() keyword and maps that to a BPF_XADD version (BPF_W or BPF_DW). In the interpreter (__bpf_prog_run()), as Eric mentioned, this maps to atomic_add() and atomic64_add(), respectively. So the struct bpf_insn prog[] you saw from sock_example.c can be regarded as one possible equivalent program section output from the compiler.
>> What you can race against is that an eBPF map can be _shared_ by >> multiple eBPF programs that are attached somewhere in the system, and >> they could all update a particular entry/counter from the map at the >> same time. > > Ok, so it does sound like eBPF needs to define/choose a memory-model and > I worry that riding on the back of __sync isn't necessarily the right > thing to do, particularly as its fallen out of favour with the compiler > folks. On weakly-ordered architectures, it's also going to result in > heavy-weight barriers for all atomic operations. > > Will > > [1] https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html
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