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powerpc/bpf: Implement support for tail calls
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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>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
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Naveen N. Rao authored and Michael Ellerman committed Oct 4, 2016
1 parent 7b847f5 commit ce07614
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Showing 4 changed files with 126 additions and 28 deletions.
2 changes: 2 additions & 0 deletions arch/powerpc/include/asm/ppc-opcode.h
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Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand All @@ -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
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2 changes: 2 additions & 0 deletions arch/powerpc/net/bpf_jit.h
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Original file line number Diff line number Diff line change
Expand Up @@ -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)
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1 change: 1 addition & 0 deletions arch/powerpc/net/bpf_jit64.h
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Original file line number Diff line number Diff line change
Expand Up @@ -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 {
/*
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149 changes: 121 additions & 28 deletions arch/powerpc/net/bpf_jit_comp64.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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"

Expand Down Expand Up @@ -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)
Expand All @@ -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)) {
/*
Expand Down Expand Up @@ -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))
Expand All @@ -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,
Expand Down Expand Up @@ -846,9 +936,12 @@ static int bpf_jit_build_body(struct bpf_prog *fp, u32 *image,
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:
/*
Expand Down

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