On 10/21/24 6:46 PM, Leon Hwang wrote:
On 22/10/24 01:49, Yonghong Song wrote:
On 10/21/24 6:39 AM, Leon Hwang wrote:
In the x86_64 JIT, when calling a function, tailcall info is
propagated if
the program is tail_call_reachable, regardless of whether the function
is a
subprog, helper, or kfunc. However, this propagation is unnecessary for
not-tail_call_reachable subprogs, helpers, or kfuncs.
The verifier can determine if a subprog is tail_call_reachable.
Therefore,
it can be optimized to only propagate tailcall info when the callee is
subprog and the subprog is actually tail_call_reachable.
Signed-off-by: Leon Hwang <leon.hwang@xxxxxxxxx>
---
arch/x86/net/bpf_jit_comp.c | 4 +++-
kernel/bpf/verifier.c | 6 ++++++
2 files changed, 9 insertions(+), 1 deletion(-)
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 06b080b61aa57..6ad6886ecfc88 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -2124,10 +2124,12 @@ st: if (is_imm8(insn->off))
/* call */
case BPF_JMP | BPF_CALL: {
+ bool pseudo_call = src_reg == BPF_PSEUDO_CALL;
+ bool subprog_tail_call_reachable = dst_reg;
u8 *ip = image + addrs[i - 1];
func = (u8 *) __bpf_call_base + imm32;
- if (tail_call_reachable) {
+ if (pseudo_call && subprog_tail_call_reachable) {
Why we need subprog_tail_call_reachable? Does
tail_call_reachable && psueudo_call
work the same way?
'tail_call_reachable && pseudo_call' works too. However, it will
propagate tailcall info to subprog even if the subprog is not
tail_call_reachable.
subprog_tail_call_reachable indicates the subprog requires tailcall info
from its caller.
So, 'pseudo_call && subprog_tail_call_reachable' is better.
In verifier.c, we have
func[i]->aux->tail_call_reachable = env->subprog_info[i].tail_call_reachable;
that is subprog_info tail_call_reachable has been transferred to func[i] tail_call_reachable.
In x86 do_jit() func, we have
bool tail_call_reachable = bpf_prog->aux->tail_call_reachable
So looks like we do not need verifier.c change here.
Did I miss anything? Could you give a concrete example to show
subprog_tail_call_reachable approach is better than tail_call_reachable?
Thanks,
Leon
