Re: [PATCH bpf-next v3 4/5] bpf, x86: Add jit support for private stack

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On 9/30/24 8:03 AM, Alexei Starovoitov wrote:
On Thu, Sep 26, 2024 at 4:45 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
Add jit support for private stack. For a particular subtree, e.g.,
   subtree_root <== stack depth 120
    subprog1    <== stack depth 80
     subprog2   <== stack depth 40
    subprog3    <== stack depth 160

Let us say that private_stack_ptr is the memory address allocated for
private stack. The frame pointer for each above is calculated like below:
   subtree_root  <== subtree_root_fp = private_stack_ptr + 120
    subprog1     <== subtree_subprog1_fp = subtree_root_fp + 80
     subprog2    <== subtree_subprog2_fp = subtree_subprog1_fp + 40
    subprog3     <== subtree_subprog1_fp = subtree_root_fp + 160

For any function call to helper/kfunc, push/pop prog frame pointer
is needed in order to preserve frame pointer value.

To deal with exception handling, push/pop frame pointer is also used
surrounding call to subsequent subprog. For example,
   subtree_root
    subprog1
      ...
      insn: call bpf_throw
      ...

After jit, we will have
   subtree_root
    insn: push r9
    subprog1
      ...
      insn: push r9
      insn: call bpf_throw
      insn: pop r9
      ...
    insn: pop r9

   exception_handler
      pop r9
      ...
where r9 represents the fp for each subprog.
Kumar,
please review the interaction of priv_stack with exceptions.

Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
---
  arch/x86/net/bpf_jit_comp.c | 87 ++++++++++++++++++++++++++++++++++---
  1 file changed, 81 insertions(+), 6 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 06b080b61aa5..c264822c926b 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -325,6 +325,22 @@ struct jit_context {
  /* Number of bytes that will be skipped on tailcall */
  #define X86_TAIL_CALL_OFFSET   (12 + ENDBR_INSN_SIZE)

+static void push_r9(u8 **pprog)
+{
+       u8 *prog = *pprog;
+
+       EMIT2(0x41, 0x51);   /* push r9 */
+       *pprog = prog;
+}
+
+static void pop_r9(u8 **pprog)
+{
+       u8 *prog = *pprog;
+
+       EMIT2(0x41, 0x59);   /* pop r9 */
+       *pprog = prog;
+}
+
  static void push_r12(u8 **pprog)
  {
         u8 *prog = *pprog;
@@ -491,7 +507,7 @@ static void emit_prologue_tail_call(u8 **pprog, bool is_subprog)
   */
  static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf,
                           bool tail_call_reachable, bool is_subprog,
-                         bool is_exception_cb)
+                         bool is_exception_cb, enum bpf_pstack_state  pstack)
enum bpf_priv_stack_mode priv_stack_mode

Okay.


  {
         u8 *prog = *pprog;

@@ -518,6 +534,8 @@ static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf,
                  * first restore those callee-saved regs from stack, before
                  * reusing the stack frame.
                  */
+               if (pstack)
+                       pop_r9(&prog);
This is an unnecessary cognitive load, since readers
need to remember absolute values of enum.
Just use
if (priv_stack_mode != NO_PRIV_STACK)

Will do.


                 pop_callee_regs(&prog, all_callee_regs_used);
                 pop_r12(&prog);
                 /* Reset the stack frame. */
@@ -1404,6 +1422,22 @@ static void emit_shiftx(u8 **pprog, u32 dst_reg, u8 src_reg, bool is64, u8 op)
         *pprog = prog;
  }

+static void emit_private_frame_ptr(u8 **pprog, void *private_frame_ptr)
+{
+       u8 *prog = *pprog;
+
+       /* movabs r9, private_frame_ptr */
+       emit_mov_imm64(&prog, X86_REG_R9, (long) private_frame_ptr >> 32,
+                      (u32) (long) private_frame_ptr);
+
+       /* add <r9>, gs:[<off>] */
+       EMIT2(0x65, 0x4c);
+       EMIT3(0x03, 0x0c, 0x25);
+       EMIT((u32)(unsigned long)&this_cpu_off, 4);
+
+       *pprog = prog;
+}
+
  #define INSN_SZ_DIFF (((addrs[i] - addrs[i - 1]) - (prog - temp)))

  #define __LOAD_TCC_PTR(off)                    \
@@ -1421,20 +1455,31 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image
         int insn_cnt = bpf_prog->len;
         bool seen_exit = false;
         u8 temp[BPF_MAX_INSN_SIZE + BPF_INSN_SAFETY];
+       void __percpu *private_frame_ptr = NULL;
         u64 arena_vm_start, user_vm_start;
+       u32 orig_stack_depth, stack_depth;
         int i, excnt = 0;
         int ilen, proglen = 0;
         u8 *prog = temp;
         int err;

+       stack_depth = bpf_prog->aux->stack_depth;
+       orig_stack_depth = round_up(stack_depth, 8);
+       if (bpf_prog->pstack) {
+               stack_depth = 0;
+               if (bpf_prog->pstack == PSTACK_TREE_ROOT)
+                       private_frame_ptr = bpf_prog->private_stack_ptr + orig_stack_depth;
+       }
Same issue.
switch (priv_stack_mode) {
case PRIV_STACK_MAIN_PROG:
     priv_frame_ptr = bpf_prog->priv_stack_ptr + orig_stack_depth;
     fallthrough;
case PRIV_STACK_SUB_PROG:
     stack_depth = 0;
     break;
}

would be easier to read.

Will do.


+
         arena_vm_start = bpf_arena_get_kern_vm_start(bpf_prog->aux->arena);
         user_vm_start = bpf_arena_get_user_vm_start(bpf_prog->aux->arena);

         detect_reg_usage(insn, insn_cnt, callee_regs_used);

-       emit_prologue(&prog, bpf_prog->aux->stack_depth,
+       emit_prologue(&prog, stack_depth,
                       bpf_prog_was_classic(bpf_prog), tail_call_reachable,
-                     bpf_is_subprog(bpf_prog), bpf_prog->aux->exception_cb);
+                     bpf_is_subprog(bpf_prog), bpf_prog->aux->exception_cb,
+                     bpf_prog->pstack);
         /* Exception callback will clobber callee regs for its own use, and
          * restore the original callee regs from main prog's stack frame.
          */
@@ -1454,6 +1499,17 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image
                 emit_mov_imm64(&prog, X86_REG_R12,
                                arena_vm_start >> 32, (u32) arena_vm_start);

+       if (bpf_prog->pstack == PSTACK_TREE_ROOT) {
+               emit_private_frame_ptr(&prog, private_frame_ptr);
+       } else if (bpf_prog->pstack == PSTACK_TREE_INTERNAL  && orig_stack_depth) {
+               /* r9 += orig_stack_depth */
+               maybe_emit_1mod(&prog, X86_REG_R9, true);
+               if (is_imm8(orig_stack_depth))
+                       EMIT3(0x83, add_1reg(0xC0, X86_REG_R9), orig_stack_depth);
+               else
+                       EMIT2_off32(0x81, add_1reg(0xC0, X86_REG_R9), orig_stack_depth);
+       }
We've been open coding 'add' insn like this for way too long.
Let's address this technical debt now.
Please move
                 case BPF_ALU | BPF_ADD | BPF_K:
                 case BPF_ALU | BPF_SUB | BPF_K:
                 case BPF_ALU | BPF_AND | BPF_K:
                 case BPF_ALU | BPF_OR | BPF_K:
                 case BPF_ALU | BPF_XOR | BPF_K:
                 case BPF_ALU64 | BPF_ADD | BPF_K:
                 case BPF_ALU64 | BPF_SUB | BPF_K:
                 case BPF_ALU64 | BPF_AND | BPF_K:
                 case BPF_ALU64 | BPF_OR | BPF_K:
                 case BPF_ALU64 | BPF_XOR | BPF_K:
into helpers and use it here.

Will do.





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