From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Yonghong Song <yhs@fb.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
Brendan Jackman <jackmanb@google.com>, bpf <bpf@vger.kernel.org>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
KP Singh <kpsingh@chromium.org>,
Florent Revest <revest@chromium.org>,
open list <linux-kernel@vger.kernel.org>,
Jann Horn <jannh@google.com>
Subject: Re: [PATCH v2 bpf-next 00/13] Atomics for eBPF
Date: Tue, 1 Dec 2020 18:00:22 -0800 [thread overview]
Message-ID: <CAEf4BzYc=c_2xCMFAE6RjMCHKWJj2euP2B21y-jfvsNzPVkhpQ@mail.gmail.com> (raw)
In-Reply-To: <4fa9f8cf-aaf8-a63c-e0ca-7d4c83b01578@fb.com>
On Mon, Nov 30, 2020 at 7:51 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> 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.
Great, this means that all existing valid uses of
__sync_fetch_and_add() will generate BPF_XADD instructions and will
work on old kernels, right?
If that's the case, do we still need cpu=v4? The new instructions are
*only* going to be generated if the user uses previously unsupported
__sync_fetch_xxx() intrinsics. So, in effect, the user consciously
opts into using new BPF instructions. cpu=v4 seems like an unnecessary
tautology then?
next prev parent reply other threads:[~2020-12-02 2:01 UTC|newest]
Thread overview: 57+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-27 17:57 [PATCH v2 bpf-next 00/13] Atomics for eBPF Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 01/13] bpf: x86: Factor out emission of ModR/M for *(reg + off) Brendan Jackman
2020-11-29 1:15 ` Alexei Starovoitov
2020-12-01 12:14 ` Brendan Jackman
2020-12-02 5:50 ` Alexei Starovoitov
2020-12-02 10:52 ` Brendan Jackman
2020-12-02 17:35 ` Alexei Starovoitov
2020-11-27 17:57 ` [PATCH v2 bpf-next 02/13] bpf: x86: Factor out emission of REX byte Brendan Jackman
2020-11-29 1:14 ` Alexei Starovoitov
2020-12-01 12:12 ` Brendan Jackman
2020-12-02 5:48 ` Alexei Starovoitov
2020-12-02 10:54 ` Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 03/13] bpf: x86: Factor out function to emit NEG Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 04/13] bpf: x86: Factor out a lookup table for some ALU opcodes Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 05/13] bpf: Rename BPF_XADD and prepare to encode other atomics in .imm Brendan Jackman
2020-11-28 3:43 ` Yonghong Song
2020-12-01 12:17 ` Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 06/13] bpf: Move BPF_STX reserved field check into BPF_STX verifier code Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 07/13] bpf: Add BPF_FETCH field / create atomic_fetch_add instruction Brendan Jackman
2020-11-28 4:15 ` Yonghong Song
2020-12-01 12:22 ` Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 08/13] bpf: Add instructions for atomic_[cmp]xchg Brendan Jackman
2020-11-28 5:25 ` Yonghong Song
2020-12-01 12:27 ` Brendan Jackman
2020-11-29 1:27 ` Alexei Starovoitov
2020-12-01 12:32 ` Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 09/13] bpf: Pull out a macro for interpreting atomic ALU operations Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 10/13] bpf: Add instructions for atomic[64]_[fetch_]sub Brendan Jackman
2020-11-27 21:39 ` kernel test robot
2020-11-27 21:39 ` [RFC PATCH] bpf: bpf_atomic_alu_string[] can be static kernel test robot
2020-11-28 5:35 ` [PATCH v2 bpf-next 10/13] bpf: Add instructions for atomic[64]_[fetch_]sub Yonghong Song
2020-11-29 1:34 ` Alexei Starovoitov
2020-11-30 17:18 ` Yonghong Song
2020-12-01 12:38 ` Brendan Jackman
2020-12-02 5:55 ` Alexei Starovoitov
2020-12-02 11:19 ` Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 11/13] bpf: Add bitwise atomic instructions Brendan Jackman
2020-11-28 5:39 ` Yonghong Song
2020-11-29 1:36 ` Alexei Starovoitov
2020-11-30 17:20 ` Yonghong Song
2020-11-27 17:57 ` [PATCH v2 bpf-next 12/13] bpf: Add tests for new BPF atomic operations Brendan Jackman
2020-12-01 3:55 ` Yonghong Song
2020-12-01 12:56 ` Brendan Jackman
2020-12-01 17:24 ` Yonghong Song
2020-12-02 2:22 ` Andrii Nakryiko
2020-12-02 12:26 ` Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 13/13] bpf: Document new atomic instructions Brendan Jackman
2020-11-28 5:53 ` [PATCH v2 bpf-next 00/13] Atomics for eBPF Yonghong Song
2020-11-29 1:40 ` Alexei Starovoitov
2020-11-30 17:22 ` Yonghong Song
2020-12-01 3:48 ` Yonghong Song
2020-12-02 2:00 ` Andrii Nakryiko [this message]
2020-12-02 5:05 ` Yonghong Song
2020-12-02 5:53 ` John Fastabend
2020-12-02 5:59 ` Andrii Nakryiko
2020-12-02 6:27 ` John Fastabend
2020-12-02 8:03 ` Yonghong Song
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