linux-kernel - Re: [PATCH bpf-next v3 09/14] bpf: Pull out a macro for interpreting atomic ALU operations

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Date:   Fri, 4 Dec 2020 09:29:13 +0000
From:   Brendan Jackman <jackmanb@...gle.com>
To:     Yonghong Song <yhs@...com>
Cc:     bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        KP Singh <kpsingh@...omium.org>,
        Florent Revest <revest@...omium.org>,
        linux-kernel@...r.kernel.org, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH bpf-next v3 09/14] bpf: Pull out a macro for interpreting
 atomic ALU operations

On Thu, Dec 03, 2020 at 10:30:18PM -0800, Yonghong Song wrote:
> 
> 
> On 12/3/20 8:02 AM, Brendan Jackman wrote:
> > Since the atomic operations that are added in subsequent commits are
> > all isomorphic with BPF_ADD, pull out a macro to avoid the
> > interpreter becoming dominated by lines of atomic-related code.
> > 
> > Note that this sacrificies interpreter performance (combining
> > STX_ATOMIC_W and STX_ATOMIC_DW into single switch case means that we
> > need an extra conditional branch to differentiate them) in favour of
> > compact and (relatively!) simple C code.
> > 
> > Change-Id: I8cae5b66e75f34393de6063b91c05a8006fdd9e6
> > Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
> 
> Ack with a minor suggestion below.
> 
> Acked-by: Yonghong Song <yhs@...com>
> 
> > ---
> >   kernel/bpf/core.c | 79 +++++++++++++++++++++++------------------------
> >   1 file changed, 38 insertions(+), 41 deletions(-)
> > 
> > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > index 28f960bc2e30..498d3f067be7 100644
> > --- a/kernel/bpf/core.c
> > +++ b/kernel/bpf/core.c
> > @@ -1618,55 +1618,52 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> >   	LDX_PROBE(DW, 8)
> >   #undef LDX_PROBE
> > -	STX_ATOMIC_W:
> > -		switch (IMM) {
> > -		case BPF_ADD:
> > -			/* lock xadd *(u32 *)(dst_reg + off16) += src_reg */
> > -			atomic_add((u32) SRC, (atomic_t *)(unsigned long)
> > -				   (DST + insn->off));
> > -			break;
> > -		case BPF_ADD | BPF_FETCH:
> > -			SRC = (u32) atomic_fetch_add(
> > -				(u32) SRC,
> > -				(atomic_t *)(unsigned long) (DST + insn->off));
> > -			break;
> > -		case BPF_XCHG:
> > -			SRC = (u32) atomic_xchg(
> > -				(atomic_t *)(unsigned long) (DST + insn->off),
> > -				(u32) SRC);
> > -			break;
> > -		case BPF_CMPXCHG:
> > -			BPF_R0 = (u32) atomic_cmpxchg(
> > -				(atomic_t *)(unsigned long) (DST + insn->off),
> > -				(u32) BPF_R0, (u32) SRC);
> > +#define ATOMIC(BOP, KOP)						\
> 
> ATOMIC a little bit generic. Maybe ATOMIC_FETCH_BOP?

Well it doesn't fetch in all cases and "BOP" is intended to
differentiate from KOP i.e. BOP = BPF operation KOP = Kernel operation.

Could go with ATOMIC_ALU_OP?

> > +		case BOP:						\
> > +			if (BPF_SIZE(insn->code) == BPF_W)		\
> > +				atomic_##KOP((u32) SRC, (atomic_t *)(unsigned long) \
> > +					     (DST + insn->off));	\
> > +			else						\
> > +				atomic64_##KOP((u64) SRC, (atomic64_t *)(unsigned long) \
> > +					       (DST + insn->off));	\
> > +			break;						\
> > +		case BOP | BPF_FETCH:					\
> > +			if (BPF_SIZE(insn->code) == BPF_W)		\
> > +				SRC = (u32) atomic_fetch_##KOP(		\
> > +					(u32) SRC,			\
> > +					(atomic_t *)(unsigned long) (DST + insn->off)); \
> > +			else						\
> > +				SRC = (u64) atomic64_fetch_##KOP(	\
> > +					(u64) SRC,			\
> > +					(atomic64_t *)(s64) (DST + insn->off)); \
> >   			break;
> > -		default:
> > -			goto default_label;
> > -		}
> > -		CONT;
> >   	STX_ATOMIC_DW:
> > +	STX_ATOMIC_W:
> >   		switch (IMM) {
> > -		case BPF_ADD:
> > -			/* lock xadd *(u64 *)(dst_reg + off16) += src_reg */
> > -			atomic64_add((u64) SRC, (atomic64_t *)(unsigned long)
> > -				     (DST + insn->off));
> > -			break;
> > -		case BPF_ADD | BPF_FETCH:
> > -			SRC = (u64) atomic64_fetch_add(
> > -				(u64) SRC,
> > -				(atomic64_t *)(s64) (DST + insn->off));
> > -			break;
> > +		ATOMIC(BPF_ADD, add)
> > +
> >   		case BPF_XCHG:
> > -			SRC = (u64) atomic64_xchg(
> > -				(atomic64_t *)(u64) (DST + insn->off),
> > -				(u64) SRC);
> > +			if (BPF_SIZE(insn->code) == BPF_W)
> > +				SRC = (u32) atomic_xchg(
> > +					(atomic_t *)(unsigned long) (DST + insn->off),
> > +					(u32) SRC);
> > +			else
> > +				SRC = (u64) atomic64_xchg(
> > +					(atomic64_t *)(u64) (DST + insn->off),
> > +					(u64) SRC);
> >   			break;
> >   		case BPF_CMPXCHG:
> > -			BPF_R0 = (u64) atomic64_cmpxchg(
> > -				(atomic64_t *)(u64) (DST + insn->off),
> > -				(u64) BPF_R0, (u64) SRC);
> > +			if (BPF_SIZE(insn->code) == BPF_W)
> > +				BPF_R0 = (u32) atomic_cmpxchg(
> > +					(atomic_t *)(unsigned long) (DST + insn->off),
> > +					(u32) BPF_R0, (u32) SRC);
> > +			else
> > +				BPF_R0 = (u64) atomic64_cmpxchg(
> > +					(atomic64_t *)(u64) (DST + insn->off),
> > +					(u64) BPF_R0, (u64) SRC);
> >   			break;
> > +
> >   		default:
> >   			goto default_label;
> >   		}
> >