From: John Fastabend <john.fastabend@gmail.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>,
John Fastabend <john.fastabend@gmail.com>
Cc: Yonghong Song <yhs@fb.com>,
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, 01 Dec 2020 22:27:48 -0800 [thread overview]
Message-ID: <5fc733e42f63f_15eb720841@john-XPS-13-9370.notmuch> (raw)
In-Reply-To: <CAEf4BzY0vHvrj4jAc+qszSMcYABd0dGFVxkM-d8S=wwHoDFD=A@mail.gmail.com>
Andrii Nakryiko wrote:
> On Tue, Dec 1, 2020 at 9:53 PM John Fastabend <john.fastabend@gmail.com> wrote:
> >
> > Yonghong Song wrote:
> > >
> > >
> >
> > [...]
> >
> > > > 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?
> > >
> > > That is correct.
> > >
> > > >
> > > > 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?
> > >
> > > This is a very good question. Essentially this boils to when users can
> > > use the new functionality including meaningful return value of
> > > __sync_fetch_and_add().
> > > (1). user can write a small bpf program to test the feature. If user
> > > gets a failed compilation (fatal error), it won't be supported.
> > > Otherwise, it is supported.
> > > (2). compiler provides some way to tell user it is safe to use, e.g.,
> > > -mcpu=v4, or some clang macro suggested by Brendan earlier.
> > >
> > > I guess since kernel already did a lot of feature discovery. Option (1)
> > > is probably fine.
> >
> > For option (2) we can use BTF with kernel version check. If kernel is
> > greater than kernel this lands in we use the the new instructions if
> > not we use a slower locked version. That should work for all cases
> > unless someone backports patches into an older case.
>
> Two different things: Clang support detection and kernel support
> detection. You are talking about kernel detection, I and Yonghong were
> talking about Clang detection and explicit cpu=v4 opt-in.
>
Ah right, catching up on email and reading the thread backwords I lost
the context thanks!
So, I live in a dev world where I control the build infrastructure so
always know clang/llvm versions and features. What I don't know as
well is where the program I just built might be run. So its a bit
of an odd question from my perspective to ask if my clang supports
feature X. If it doesn't support feature X and I want it we upgrade
clang so that it does support it. I don't think we would ever
write a program to test the assertion. Anyways thanks.
> For kernel detection, if there is an enum value or type that gets
> added along the feature, then with CO-RE built-ins it's easy to detect
> and kernel dead code elimination will make sure that unsupported
> instructions won't trip up the BPF verifier. Still need Clang support
> to compile the program in the first place, though.
+1
>
> If there is no such BTF-based way to check, it is still possible to
> try to load a trivial BPF program with __sync_fech_and_xxx() to do
> feature detection and then use .rodata to turn off code paths relying
> on a new instruction set.
Right.
>
> >
> > At least thats what I'll probably end up wrapping in a helper function.
next prev parent reply other threads:[~2020-12-02 6:28 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
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 [this message]
2020-12-02 8:03 ` Yonghong Song
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