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Subject[PATCH 2/3] bpf powerpc: implement support for tail calls
Date
Tail calls allow JIT'ed eBPF programs to call into other JIT'ed eBPF
programs. This can be achieved either by:
(1) retaining the stack setup by the first eBPF program and having all
subsequent eBPF programs re-using it, or,
(2) by unwinding/tearing down the stack and having each eBPF program
deal with its own stack as it sees fit.

To ensure that this does not create loops, there is a limit to how many
tail calls can be done (currently 32). This requires the JIT'ed code to
maintain a count of the number of tail calls done so far.

Approach (1) is simple, but requires every eBPF program to have (almost)
the same prologue/epilogue, regardless of whether they need it. This is
inefficient for small eBPF programs which may not sometimes need a
prologue at all. As such, to minimize impact of tail call
implementation, we use approach (2) here which needs each eBPF program
in the chain to use its own prologue/epilogue. This is not ideal when
many tail calls are involved and when all the eBPF programs in the chain
have similar prologue/epilogue. However, the impact is restricted to
programs that do tail calls. Individual eBPF programs are not affected.

We maintain the tail call count in a fixed location on the stack and
updated tail call count values are passed in through this. The very
first eBPF program in a chain sets this up to 0 (the first 2
instructions). Subsequent tail calls skip the first two eBPF JIT
instructions to maintain the count. For programs that don't do tail
calls themselves, the first two instructions are NOPs.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
---
arch/powerpc/include/asm/ppc-opcode.h | 2 +
arch/powerpc/net/bpf_jit.h | 2 +
arch/powerpc/net/bpf_jit64.h | 1 +
arch/powerpc/net/bpf_jit_comp64.c | 149 +++++++++++++++++++++++++++-------
4 files changed, 126 insertions(+), 28 deletions(-)

diff --git a/arch/powerpc/include/asm/ppc-opcode.h b/arch/powerpc/include/asm/ppc-opcode.h
index 127ebf5..54ff8ce 100644
--- a/arch/powerpc/include/asm/ppc-opcode.h
+++ b/arch/powerpc/include/asm/ppc-opcode.h
@@ -236,6 +236,7 @@
#define PPC_INST_STWU 0x94000000
#define PPC_INST_MFLR 0x7c0802a6
#define PPC_INST_MTLR 0x7c0803a6
+#define PPC_INST_MTCTR 0x7c0903a6
#define PPC_INST_CMPWI 0x2c000000
#define PPC_INST_CMPDI 0x2c200000
#define PPC_INST_CMPW 0x7c000000
@@ -250,6 +251,7 @@
#define PPC_INST_SUB 0x7c000050
#define PPC_INST_BLR 0x4e800020
#define PPC_INST_BLRL 0x4e800021
+#define PPC_INST_BCTR 0x4e800420
#define PPC_INST_MULLD 0x7c0001d2
#define PPC_INST_MULLW 0x7c0001d6
#define PPC_INST_MULHWU 0x7c000016
diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
index d5301b6..89f7007 100644
--- a/arch/powerpc/net/bpf_jit.h
+++ b/arch/powerpc/net/bpf_jit.h
@@ -40,6 +40,8 @@
#define PPC_BLR() EMIT(PPC_INST_BLR)
#define PPC_BLRL() EMIT(PPC_INST_BLRL)
#define PPC_MTLR(r) EMIT(PPC_INST_MTLR | ___PPC_RT(r))
+#define PPC_BCTR() EMIT(PPC_INST_BCTR)
+#define PPC_MTCTR(r) EMIT(PPC_INST_MTCTR | ___PPC_RT(r))
#define PPC_ADDI(d, a, i) EMIT(PPC_INST_ADDI | ___PPC_RT(d) | \
___PPC_RA(a) | IMM_L(i))
#define PPC_MR(d, a) PPC_OR(d, a, a)
diff --git a/arch/powerpc/net/bpf_jit64.h b/arch/powerpc/net/bpf_jit64.h
index a1645d7..038e00b 100644
--- a/arch/powerpc/net/bpf_jit64.h
+++ b/arch/powerpc/net/bpf_jit64.h
@@ -88,6 +88,7 @@ DECLARE_LOAD_FUNC(sk_load_byte);
#define SEEN_FUNC 0x1000 /* might call external helpers */
#define SEEN_STACK 0x2000 /* uses BPF stack */
#define SEEN_SKB 0x4000 /* uses sk_buff */
+#define SEEN_TAILCALL 0x8000 /* uses tail calls */

struct codegen_context {
/*
diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
index 5f8c91f..3ec29d6 100644
--- a/arch/powerpc/net/bpf_jit_comp64.c
+++ b/arch/powerpc/net/bpf_jit_comp64.c
@@ -17,6 +17,7 @@
#include <linux/filter.h>
#include <linux/if_vlan.h>
#include <asm/kprobes.h>
+#include <linux/bpf.h>

#include "bpf_jit64.h"

@@ -77,6 +78,11 @@ static int bpf_jit_stack_local(struct codegen_context *ctx)
return -(BPF_PPC_STACK_SAVE + 16);
}

+static int bpf_jit_stack_tailcallcnt(struct codegen_context *ctx)
+{
+ return bpf_jit_stack_local(ctx) + 8;
+}
+
static int bpf_jit_stack_offsetof(struct codegen_context *ctx, int reg)
{
if (reg >= BPF_PPC_NVR_MIN && reg < 32)
@@ -102,33 +108,25 @@ static void bpf_jit_emit_skb_loads(u32 *image, struct codegen_context *ctx)
PPC_BPF_LL(b2p[SKB_DATA_REG], 3, offsetof(struct sk_buff, data));
}

-static void bpf_jit_emit_func_call(u32 *image, struct codegen_context *ctx, u64 func)
+static void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
{
-#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]);
+ int i;
+
/*
- * 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.
+ * Initialize tail_call_cnt if we do tail calls.
+ * Otherwise, put in NOPs so that it can be skipped when we are
+ * invoked through a tail call.
*/
- PPC_BPF_LL(2, b2p[TMP_REG_2], 8);
-#else
- /* We can clobber r12 */
- PPC_FUNC_ADDR(12, func);
- PPC_MTLR(12);
-#endif
- PPC_BLRL();
-}
+ if (ctx->seen & SEEN_TAILCALL) {
+ PPC_LI(b2p[TMP_REG_1], 0);
+ /* this goes in the redzone */
+ PPC_BPF_STL(b2p[TMP_REG_1], 1, -(BPF_PPC_STACK_SAVE + 8));
+ } else {
+ PPC_NOP();
+ PPC_NOP();
+ }

-static void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
-{
- int i;
+#define BPF_TAILCALL_PROLOGUE_SIZE 8

if (bpf_has_stack_frame(ctx)) {
/*
@@ -170,13 +168,10 @@ static void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
STACK_FRAME_MIN_SIZE + MAX_BPF_STACK);
}

-static void bpf_jit_build_epilogue(u32 *image, struct codegen_context *ctx)
+static void bpf_jit_emit_common_epilogue(u32 *image, struct codegen_context *ctx)
{
int i;

- /* Move result to r3 */
- PPC_MR(3, b2p[BPF_REG_0]);
-
/* Restore NVRs */
for (i = BPF_REG_6; i <= BPF_REG_10; i++)
if (bpf_is_seen_register(ctx, i))
@@ -198,10 +193,105 @@ static void bpf_jit_build_epilogue(u32 *image, struct codegen_context *ctx)
PPC_MTLR(0);
}
}
+}
+
+static void bpf_jit_build_epilogue(u32 *image, struct codegen_context *ctx)
+{
+ bpf_jit_emit_common_epilogue(image, ctx);
+
+ /* Move result to r3 */
+ PPC_MR(3, b2p[BPF_REG_0]);

PPC_BLR();
}

+static void bpf_jit_emit_func_call(u32 *image, struct codegen_context *ctx, u64 func)
+{
+#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);
+#endif
+ PPC_BLRL();
+}
+
+static void bpf_jit_emit_tail_call(u32 *image, struct codegen_context *ctx, u32 out)
+{
+ /*
+ * By now, the eBPF program has already setup parameters in r3, r4 and r5
+ * r3/BPF_REG_1 - pointer to ctx -- passed as is to the next bpf program
+ * r4/BPF_REG_2 - pointer to bpf_array
+ * r5/BPF_REG_3 - index in bpf_array
+ */
+ int b2p_bpf_array = b2p[BPF_REG_2];
+ int b2p_index = b2p[BPF_REG_3];
+
+ /*
+ * if (index >= array->map.max_entries)
+ * goto out;
+ */
+ PPC_LWZ(b2p[TMP_REG_1], b2p_bpf_array, offsetof(struct bpf_array, map.max_entries));
+ PPC_CMPLW(b2p_index, b2p[TMP_REG_1]);
+ PPC_BCC(COND_GE, out);
+
+ /*
+ * if (tail_call_cnt > MAX_TAIL_CALL_CNT)
+ * goto out;
+ */
+ PPC_LD(b2p[TMP_REG_1], 1, bpf_jit_stack_tailcallcnt(ctx));
+ PPC_CMPLWI(b2p[TMP_REG_1], MAX_TAIL_CALL_CNT);
+ PPC_BCC(COND_GT, out);
+
+ /*
+ * tail_call_cnt++;
+ */
+ PPC_ADDI(b2p[TMP_REG_1], b2p[TMP_REG_1], 1);
+ PPC_BPF_STL(b2p[TMP_REG_1], 1, bpf_jit_stack_tailcallcnt(ctx));
+
+ /* prog = array->ptrs[index]; */
+ PPC_MULI(b2p[TMP_REG_1], b2p_index, 8);
+ PPC_ADD(b2p[TMP_REG_1], b2p[TMP_REG_1], b2p_bpf_array);
+ PPC_LD(b2p[TMP_REG_1], b2p[TMP_REG_1], offsetof(struct bpf_array, ptrs));
+
+ /*
+ * if (prog == NULL)
+ * goto out;
+ */
+ PPC_CMPLDI(b2p[TMP_REG_1], 0);
+ PPC_BCC(COND_EQ, out);
+
+ /* goto *(prog->bpf_func + prologue_size); */
+ PPC_LD(b2p[TMP_REG_1], b2p[TMP_REG_1], offsetof(struct bpf_prog, bpf_func));
+#ifdef PPC64_ELF_ABI_v1
+ /* skip past the function descriptor */
+ PPC_ADDI(b2p[TMP_REG_1], b2p[TMP_REG_1],
+ FUNCTION_DESCR_SIZE + BPF_TAILCALL_PROLOGUE_SIZE);
+#else
+ PPC_ADDI(b2p[TMP_REG_1], b2p[TMP_REG_1], BPF_TAILCALL_PROLOGUE_SIZE);
+#endif
+ PPC_MTCTR(b2p[TMP_REG_1]);
+
+ /* tear down stack, restore NVRs, ... */
+ bpf_jit_emit_common_epilogue(image, ctx);
+
+ PPC_BCTR();
+ /* out: */
+}
+
/* Assemble the body code between the prologue & epilogue */
static int bpf_jit_build_body(struct bpf_prog *fp, u32 *image,
struct codegen_context *ctx,
@@ -846,9 +936,12 @@ common_load:
break;

/*
- * TODO: Tail call
+ * Tail call
*/
case BPF_JMP | BPF_CALL | BPF_X:
+ ctx->seen |= SEEN_TAILCALL;
+ bpf_jit_emit_tail_call(image, ctx, addrs[i + 1]);
+ break;

default:
/*
--
2.9.3
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