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SubjectRe: [PATCH v2 bpf-next 08/13] bpf: Add instructions for atomic_[cmp]xchg
On Fri, Nov 27, 2020 at 09:25:53PM -0800, Yonghong Song wrote:
>
>
> On 11/27/20 9:57 AM, Brendan Jackman wrote:
> > This adds two atomic opcodes, both of which include the BPF_FETCH
> > flag. XCHG without the BPF_FETCh flag would naturally encode
>
> BPF_FETCH

Ack, thanks

> > atomic_set. This is not supported because it would be of limited
> > value to userspace (it doesn't imply any barriers). CMPXCHG without
> > BPF_FETCH woulud be an atomic compare-and-write. We don't have such
> > an operation in the kernel so it isn't provided to BPF either.
> >
> > There are two significant design decisions made for the CMPXCHG
> > instruction:
> >
> > - To solve the issue that this operation fundamentally has 3
> > operands, but we only have two register fields. Therefore the
> > operand we compare against (the kernel's API calls it 'old') is
> > hard-coded to be R0. x86 has similar design (and A64 doesn't
> > have this problem).
> >
> > A potential alternative might be to encode the other operand's
> > register number in the immediate field.
> >
> > - The kernel's atomic_cmpxchg returns the old value, while the C11
> > userspace APIs return a boolean indicating the comparison
> > result. Which should BPF do? A64 returns the old value. x86 returns
> > the old value in the hard-coded register (and also sets a
> > flag). That means return-old-value is easier to JIT.
> >
> > Signed-off-by: Brendan Jackman <jackmanb@google.com>
> > ---
> > arch/x86/net/bpf_jit_comp.c | 8 ++++++++
> > include/linux/filter.h | 20 ++++++++++++++++++++
> > include/uapi/linux/bpf.h | 4 +++-
> > kernel/bpf/core.c | 20 ++++++++++++++++++++
> > kernel/bpf/disasm.c | 15 +++++++++++++++
> > kernel/bpf/verifier.c | 19 +++++++++++++++++--
> > tools/include/linux/filter.h | 20 ++++++++++++++++++++
> > tools/include/uapi/linux/bpf.h | 4 +++-
> > 8 files changed, 106 insertions(+), 4 deletions(-)
> >
> [...]
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index cd4c03b25573..c8311cc114ec 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -3601,10 +3601,13 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
> > static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_insn *insn)
> > {
> > int err;
> > + int load_reg;
> > switch (insn->imm) {
> > case BPF_ADD:
> > case BPF_ADD | BPF_FETCH:
> > + case BPF_XCHG:
> > + case BPF_CMPXCHG:
> > break;
> > default:
> > verbose(env, "BPF_ATOMIC uses invalid atomic opcode %02x\n", insn->imm);
> > @@ -3626,6 +3629,13 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
> > if (err)
> > return err;
> > + if (insn->imm == BPF_CMPXCHG) {
> > + /* check src3 operand */
>
> better comment about what src3 means here?

Ack, adding "Check comparison of R0 with memory location"

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