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

[Yonghong Song]

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@xxxxxxxxxx>
> > > ---
> > >    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.

>