Re: [PATCH bpf-next v3 10/14] bpf: Add bitwise atomic instructions

[Yonghong Song <yhs@fb.com>]

On 12/4/20 1:36 AM, Brendan Jackman wrote:
> On Thu, Dec 03, 2020 at 10:42:19PM -0800, Yonghong Song wrote:
>>
>>
>> On 12/3/20 8:02 AM, Brendan Jackman wrote:
>>> This adds instructions for
>>>
>>> atomic[64]_[fetch_]and
>>> atomic[64]_[fetch_]or
>>> atomic[64]_[fetch_]xor
>>>
>>> All these operations are isomorphic enough to implement with the same
>>> verifier, interpreter, and x86 JIT code, hence being a single commit.
>>>
>>> The main interesting thing here is that x86 doesn't directly support
>>> the fetch_ version these operations, so we need to generate a CMPXCHG
>>> loop in the JIT. This requires the use of two temporary registers,
>>> IIUC it's safe to use BPF_REG_AX and x86's AUX_REG for this purpose.
>>>
>>> Change-Id: I340b10cecebea8cb8a52e3606010cde547a10ed4
>>> Signed-off-by: Brendan Jackman <jackmanb@google.com>
>>> ---
>>>    arch/x86/net/bpf_jit_comp.c  | 50 +++++++++++++++++++++++++++++-
>>>    include/linux/filter.h       | 60 ++++++++++++++++++++++++++++++++++++
>>>    kernel/bpf/core.c            |  5 ++-
>>>    kernel/bpf/disasm.c          | 21 ++++++++++---
>>>    kernel/bpf/verifier.c        |  6 ++++
>>>    tools/include/linux/filter.h | 60 ++++++++++++++++++++++++++++++++++++
>>>    6 files changed, 196 insertions(+), 6 deletions(-)
>>>
> [...]
>>> diff --git a/include/linux/filter.h b/include/linux/filter.h
>>> index 6186280715ed..698f82897b0d 100644
>>> --- a/include/linux/filter.h
>>> +++ b/include/linux/filter.h
>>> @@ -280,6 +280,66 @@ static inline bool insn_is_zext(const struct bpf_insn *insn)
> [...]
>>> +#define BPF_ATOMIC_FETCH_XOR(SIZE, DST, SRC, OFF)		\
>>> +	((struct bpf_insn) {					\
>>> +		.code  = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC,	\
>>> +		.dst_reg = DST,					\
>>> +		.src_reg = SRC,					\
>>> +		.off   = OFF,					\
>>> +		.imm   = BPF_XOR | BPF_FETCH })
>>> +
>>>    /* Atomic exchange, src_reg = atomic_xchg((dst_reg + off), src_reg) */
>>
>> Looks like BPF_ATOMIC_XOR/OR/AND/... all similar to each other.
>> The same is for BPF_ATOMIC_FETCH_XOR/OR/AND/...
>>
>> I am wondering whether it makes sence to have to
>> BPF_ATOMIC_BOP(BOP, SIZE, DST, SRC, OFF) and
>> BPF_ATOMIC_FETCH_BOP(BOP, SIZE, DST, SRC, OFF)
>> can have less number of macros?
> 
> Hmm yeah I think that's probably a good idea, it would be consistent
> with the macros for non-atomic ALU ops.
> 
> I don't think 'BOP' would be very clear though, 'ALU' might be more
> obvious.

BPF_ATOMIC_ALU and BPF_ATOMIC_FETCH_ALU indeed better.

>