On 05/24/2018 01:04 PM, Daniel Borkmann wrote:
> 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.
>
Ah, true. Thanks for pointing this out. There are a few other things that
we are planning to do for the ppc64 JIT compiler. Will put out a patch for
this with that series.
- Sandipan
>> #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();
>> }
>>
>>
>
