netdev - Re: [PATCH net-next v2 5/5] net: filter: optimize BPF migration for ARG1/CTX handling

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Date:	Thu, 24 Apr 2014 08:43:39 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2 5/5] net: filter: optimize BPF migration for
 ARG1/CTX handling

On Wed, Apr 23, 2014 at 11:45 PM, Daniel Borkmann <dborkman@...hat.com> wrote:
> Currently, at initial setup in __sk_run_filter() we initialize the
> BPF stack's frame-pointer and CTX register. However, instead of the
> CTX register, we initialize context to ARG1, and during user filter
> migration we emit *always* an instruction that copies the content
> from ARG1 over to CTX. ARG1 is needed in BPF_CALL instructions to
> setup ctx; for user BPF filter ARG2 has A, and ARG3 X for call
> emission. However, we nevertheless copy CTX over to ARG1 in these
> cases for user migrated filters. We can spare us this extra interpreter
> instruction and assign it during initial setup time.
>
> Signed-off-by: Daniel Borkmann <dborkman@...hat.com>
> Cc: Alexei Starovoitov <ast@...mgrid.com>

Acked-by: Alexei Starovoitov <ast@...mgrid.com>

> opinion on this particular patch, just wanted to have clarified what we
> have with ABI here as there's no particular freeze from the code we have

yes. Of course, we do not have a freeze on ABI.
I was arguing about merits of not changing it for this particular case.
Especially when the difference is a single line of code.
If we want to add another register to ISA, that would be a fine reason
to change it.

Thanks!

> ---
>  net/core/filter.c | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index eada3d5..9744365 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -257,7 +257,10 @@ unsigned int __sk_run_filter(void *ctx, const struct sock_filter_int *insn)
>  #define CONT_JMP ({ insn++; goto select_insn; })
>
>         FP = (u64) (unsigned long) &stack[ARRAY_SIZE(stack)];
> -       ARG1 = (u64) (unsigned long) ctx;
> +       CTX = (u64) (unsigned long) ctx;
> +
> +       /* Direct users expect arg1 to be filled already with ctx. */
> +       ARG1 = CTX;
>
>         /* Register for user BPF programs need to be reset first. */
>         regs[BPF_REG_A] = 0;
> @@ -896,13 +899,6 @@ do_pass:
>         new_insn = new_prog;
>         fp = prog;
>
> -       if (new_insn) {
> -               new_insn->code = BPF_ALU64 | BPF_MOV | BPF_X;
> -               new_insn->a_reg = BPF_REG_CTX;
> -               new_insn->x_reg = BPF_REG_ARG1;
> -       }
> -       new_insn++;
> -
>         for (i = 0; i < len; fp++, i++) {
>                 struct sock_filter_int tmp_insns[6] = { };
>                 struct sock_filter_int *insn = tmp_insns;
> --
> 1.7.11.7
>
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