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

[Yonghong Song <yhs@fb.com>]

On 12/1/20 6:00 PM, Andrii Nakryiko wrote:
> 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?

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.