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Date:   Tue, 01 Dec 2020 22:27:48 -0800
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>,
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        KP Singh <kpsingh@chromium.org>,
        Florent Revest <revest@chromium.org>,
        open list <linux-kernel@vger.kernel.org>,
        Jann Horn <jannh@google.com>
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Subject: Re: [PATCH v2 bpf-next 00/13] Atomics for eBPF
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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.