On 05/24/2018 08:56 AM, Sandipan Das wrote:
> For multi-function programs, loading the address of a callee
> function to a register requires emitting instructions whose
> count varies from one to five depending on the nature of the
> address.
>
> Since we come to know of the callee's address only before the
> extra pass, the number of instructions required to load this
> address may vary from what was previously generated. This can
> make the JITed image grow or shrink.
>
> To avoid this, we should generate a constant five-instruction
> when loading function addresses by padding the optimized load
> sequence with NOPs.
>
> Signed-off-by: Sandipan Das <sandi...@linux.vnet.ibm.com>
> ---
> arch/powerpc/net/bpf_jit_comp64.c | 34 +++++++++++++++++++++++-----------
> 1 file changed, 23 insertions(+), 11 deletions(-)
>
> diff --git a/arch/powerpc/net/bpf_jit_comp64.c
> b/arch/powerpc/net/bpf_jit_comp64.c
> index 1bdb1aff0619..e4582744a31d 100644
> --- a/arch/powerpc/net/bpf_jit_comp64.c
> +++ b/arch/powerpc/net/bpf_jit_comp64.c
> @@ -167,25 +167,37 @@ static void bpf_jit_build_epilogue(u32 *image, struct
> codegen_context *ctx)
>
> static void bpf_jit_emit_func_call(u32 *image, struct codegen_context *ctx,
> u64 func)
> {
> + unsigned int i, ctx_idx = ctx->idx;
> +
> + /* Load function address into r12 */
> + PPC_LI64(12, func);
> +
> + /* For bpf-to-bpf function calls, the callee's address is unknown
> + * until the last extra pass. As seen above, we use PPC_LI64() to
> + * load the callee's address, but this may optimize the number of
> + * instructions required based on the nature of the address.
> + *
> + * Since we don't want the number of instructions emitted to change,
> + * we pad the optimized PPC_LI64() call with NOPs to guarantee that
> + * we always have a five-instruction sequence, which is the maximum
> + * that PPC_LI64() can emit.
> + */
> + for (i = ctx->idx - ctx_idx; i < 5; i++)
> + PPC_NOP();
By the way, I think you can still optimize this. The nops are not really
needed in case of insn->src_reg != BPF_PSEUDO_CALL since the address of
a normal BPF helper call will always be at a fixed location and known a
priori.
> #ifdef PPC64_ELF_ABI_v1
> - /* func points to the function descriptor */
> - PPC_LI64(b2p[TMP_REG_2], func);
> - /* Load actual entry point from function descriptor */
> - PPC_BPF_LL(b2p[TMP_REG_1], b2p[TMP_REG_2], 0);
> - /* ... and move it to LR */
> - PPC_MTLR(b2p[TMP_REG_1]);
> /*
> * Load TOC from function descriptor at offset 8.
> * We can clobber r2 since we get called through a
> * function pointer (so caller will save/restore r2)
> * and since we don't use a TOC ourself.
> */
> - PPC_BPF_LL(2, b2p[TMP_REG_2], 8);
> -#else
> - /* We can clobber r12 */
> - PPC_FUNC_ADDR(12, func);
> - PPC_MTLR(12);
> + PPC_BPF_LL(2, 12, 8);
> + /* Load actual entry point from function descriptor */
> + PPC_BPF_LL(12, 12, 0);
> #endif
> +
> + PPC_MTLR(12);
> PPC_BLRL();
> }
>
>
