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powerpc64/bpf: Moving tail_call_cnt to bottom of frame

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To support tailcalls in subprogs, tail_call_cnt needs to be on the BPF
trampoline stack frame. In a regular BPF program or subprog stack
frame, the position of tail_call_cnt is after the NVR save area
(BPF_PPC_STACK_SAVE). To avoid complex logic in deducing offset for
tail_call_cnt, it has to be kept at the same offset on the trampoline
frame as well. But doing that wastes nearly all of BPF_PPC_STACK_SAVE
bytes on the BPF trampoline stack frame as the NVR save area is not
the same for BPF trampoline and regular BPF programs. Address this by
moving tail_call_cnt to the bottom of the frame.

This change avoids the need to account for BPF_PPC_STACK_SAVE bytes
in the BPF trampoline stack frame when support for tailcalls in BPF
subprogs is added later. Also, this change makes offset calculation
of tail_call_cnt field simpler all across.

Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260124075223.6033-2-adubey@linux.ibm.com
This commit is contained in:
Abhishek Dubey 2026-01-24 13:22:18 +05:30 • committed by Madhavan Srinivasan
parent 815a8d2feb
commit 15513beeb6
3 changed files with 24 additions and 14 deletions
Download patch file Download diff file Expand all files Collapse all files

1
arch/powerpc/net/bpf_jit.h
View file

@ -24,6 +24,7 @@
#define SZL sizeof(unsigned long)
#define BPF_INSN_SAFETY 64
#define BPF_PPC_TAILCALL 8
#define PLANT_INSTR(d, idx, instr) \
do { if (d) { (d)[idx] = instr; } idx++; } while (0)

6
arch/powerpc/net/bpf_jit_comp.c
View file

@ -621,8 +621,8 @@ static void bpf_trampoline_setup_tail_call_cnt(u32 *image, struct codegen_contex
int func_frame_offset, int r4_off)
{
if (IS_ENABLED(CONFIG_PPC64)) {
/* See bpf_jit_stack_tailcallcnt() */
int tailcallcnt_offset = 7 * 8;
/* See Generated stack layout */
int tailcallcnt_offset = BPF_PPC_TAILCALL;
EMIT(PPC_RAW_LL(_R3, _R1, func_frame_offset - tailcallcnt_offset));
EMIT(PPC_RAW_STL(_R3, _R1, -tailcallcnt_offset));
@ -637,7 +637,7 @@ static void bpf_trampoline_restore_tail_call_cnt(u32 *image, struct codegen_cont
{
if (IS_ENABLED(CONFIG_PPC64)) {
/* See bpf_jit_stack_tailcallcnt() */
int tailcallcnt_offset = 7 * 8;
int tailcallcnt_offset = BPF_PPC_TAILCALL;
EMIT(PPC_RAW_LL(_R3, _R1, -tailcallcnt_offset));
EMIT(PPC_RAW_STL(_R3, _R1, func_frame_offset - tailcallcnt_offset));

31
arch/powerpc/net/bpf_jit_comp64.c
View file

@ -20,13 +20,15 @@
#include "bpf_jit.h"
/*
* Stack layout:
* Stack layout with frame:
* Layout when setting up our own stack frame.
* Note: r1 at bottom, component offsets positive wrt r1.
* Ensure the top half (upto local_tmp_var) stays consistent
* with our redzone usage.
*
* [ prev sp ] <-------------
* [ nv gpr save area ] 6*8 |
* [ tail_call_cnt ] 8 |
* [ nv gpr save area ] 6*8 |
* [ local_tmp_var ] 24 |
* fp (r31) --> [ ebpf stack space ] upto 512 |
* [ frame header ] 32/112 |
@ -36,10 +38,12 @@
/* for gpr non volatile registers BPG_REG_6 to 10 */
#define BPF_PPC_STACK_SAVE (6*8)
/* for bpf JIT code internal usage */
#define BPF_PPC_STACK_LOCALS 32
#define BPF_PPC_STACK_LOCALS 24
/* stack frame excluding BPF stack, ensure this is quadword aligned */
#define BPF_PPC_STACKFRAME (STACK_FRAME_MIN_SIZE + \
BPF_PPC_STACK_LOCALS + BPF_PPC_STACK_SAVE)
BPF_PPC_STACK_LOCALS + \
BPF_PPC_STACK_SAVE + \
BPF_PPC_TAILCALL)
/* BPF register usage */
#define TMP_REG_1 (MAX_BPF_JIT_REG + 0)
@ -87,27 +91,32 @@ static inline bool bpf_has_stack_frame(struct codegen_context *ctx)
}
/*
* Stack layout with redzone:
* When not setting up our own stackframe, the redzone (288 bytes) usage is:
* Note: r1 from prev frame. Component offset negative wrt r1.
*
* [ prev sp ] <-------------
* [ ... ] |
* sp (r1) ---> [ stack pointer ] --------------
* [ nv gpr save area ] 6*8
* [ tail_call_cnt ] 8
* [ nv gpr save area ] 6*8
* [ local_tmp_var ] 24
* [ unused red zone ] 224
*/
static int bpf_jit_stack_local(struct codegen_context *ctx)
{
if (bpf_has_stack_frame(ctx))
if (bpf_has_stack_frame(ctx)) {
/* Stack layout with frame */
return STACK_FRAME_MIN_SIZE + ctx->stack_size;
else
return -(BPF_PPC_STACK_SAVE + 32);
} else {
/* Stack layout with redzone */
return -(BPF_PPC_TAILCALL + BPF_PPC_STACK_SAVE + BPF_PPC_STACK_LOCALS);
}
}
static int bpf_jit_stack_tailcallcnt(struct codegen_context *ctx)
{
return bpf_jit_stack_local(ctx) + 24;
return bpf_jit_stack_local(ctx) + BPF_PPC_STACK_LOCALS + BPF_PPC_STACK_SAVE;
}
static int bpf_jit_stack_offsetof(struct codegen_context *ctx, int reg)
@ -115,7 +124,7 @@ static int bpf_jit_stack_offsetof(struct codegen_context *ctx, int reg)
if (reg >= BPF_PPC_NVR_MIN && reg < 32)
return (bpf_has_stack_frame(ctx) ?
(BPF_PPC_STACKFRAME + ctx->stack_size) : 0)
- (8 * (32 - reg));
- (8 * (32 - reg)) - BPF_PPC_TAILCALL;
pr_err("BPF JIT is asking about unknown registers");
BUG();
@ -145,7 +154,7 @@ void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
if (ctx->seen & SEEN_TAILCALL) {
EMIT(PPC_RAW_LI(bpf_to_ppc(TMP_REG_1), 0));
/* this goes in the redzone */
EMIT(PPC_RAW_STD(bpf_to_ppc(TMP_REG_1), _R1, -(BPF_PPC_STACK_SAVE + 8)));
EMIT(PPC_RAW_STD(bpf_to_ppc(TMP_REG_1), _R1, -(BPF_PPC_TAILCALL)));
} else {
EMIT(PPC_RAW_NOP());
EMIT(PPC_RAW_NOP());