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s390/bpf: implement bpf_tail_call() helper
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bpf_tail_call() arguments:

 - ctx......: Context pointer
 - jmp_table: One of BPF_MAP_TYPE_PROG_ARRAY maps used as the jump table
 - index....: Index in the jump table

In this implementation s390x JIT does stack unwinding and jumps into the
callee program prologue. Caller and callee use the same stack.

With this patch a tail call generates the following code on s390x:

 if (index >= array->map.max_entries)
         goto out
 000003ff8001c7e4: e31030100016   llgf    %r1,16(%r3)
 000003ff8001c7ea: ec41001fa065   clgrj   %r4,%r1,10,3ff8001c828

 if (tail_call_cnt++ > MAX_TAIL_CALL_CNT)
         goto out;
 000003ff8001c7f0: a7080001       lhi     %r0,1
 000003ff8001c7f4: eb10f25000fa   laal    %r1,%r0,592(%r15)
 000003ff8001c7fa: ec120017207f   clij    %r1,32,2,3ff8001c828

 prog = array->prog[index];
 if (prog == NULL)
         goto out;
 000003ff8001c800: eb140003000d   sllg    %r1,%r4,3
 000003ff8001c806: e31310800004   lg      %r1,128(%r3,%r1)
 000003ff8001c80c: ec18000e007d   clgij   %r1,0,8,3ff8001c828

 Restore registers before calling function
 000003ff8001c812: eb68f2980004   lmg     %r6,%r8,664(%r15)
 000003ff8001c818: ebbff2c00004   lmg     %r11,%r15,704(%r15)

 goto *(prog->bpf_func + tail_call_start);
 000003ff8001c81e: e31100200004   lg      %r1,32(%r1,%r0)
 000003ff8001c824: 47f01006       bc      15,6(%r1)

Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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Michael Holzheu authored and David S. Miller committed Jun 9, 2015
1 parent 941742f commit 6651ee0
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Showing 2 changed files with 112 additions and 4 deletions.
10 changes: 8 additions & 2 deletions arch/s390/net/bpf_jit.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -28,6 +28,9 @@ extern u8 sk_load_word[], sk_load_half[], sk_load_byte[];
* | old backchain | |
* +---------------+ |
* | r15 - r6 | |
* +---------------+ |
* | 4 byte align | |
* | tail_call_cnt | |
* BFP -> +===============+ |
* | | |
* | BPF stack | |
Expand All @@ -46,14 +49,17 @@ extern u8 sk_load_word[], sk_load_half[], sk_load_byte[];
* R15 -> +---------------+ + low
*
* We get 160 bytes stack space from calling function, but only use
* 11 * 8 byte (old backchain + r15 - r6) for storing registers.
* 12 * 8 byte for old backchain, r15..r6, and tail_call_cnt.
*/
#define STK_SPACE (MAX_BPF_STACK + 8 + 4 + 4 + 160)
#define STK_160_UNUSED (160 - 11 * 8)
#define STK_160_UNUSED (160 - 12 * 8)
#define STK_OFF (STK_SPACE - STK_160_UNUSED)
#define STK_OFF_TMP 160 /* Offset of tmp buffer on stack */
#define STK_OFF_HLEN 168 /* Offset of SKB header length on stack */

#define STK_OFF_R6 (160 - 11 * 8) /* Offset of r6 on stack */
#define STK_OFF_TCCNT (160 - 12 * 8) /* Offset of tail_call_cnt on stack */

/* Offset to skip condition code check */
#define OFF_OK 4

Expand Down
106 changes: 104 additions & 2 deletions arch/s390/net/bpf_jit_comp.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -21,6 +21,7 @@
#include <linux/netdevice.h>
#include <linux/filter.h>
#include <linux/init.h>
#include <linux/bpf.h>
#include <asm/cacheflush.h>
#include <asm/dis.h>
#include "bpf_jit.h"
Expand All @@ -40,6 +41,8 @@ struct bpf_jit {
int base_ip; /* Base address for literal pool */
int ret0_ip; /* Address of return 0 */
int exit_ip; /* Address of exit */
int tail_call_start; /* Tail call start offset */
int labels[1]; /* Labels for local jumps */
};

#define BPF_SIZE_MAX 4096 /* Max size for program */
Expand All @@ -49,6 +52,7 @@ struct bpf_jit {
#define SEEN_RET0 4 /* ret0_ip points to a valid return 0 */
#define SEEN_LITERAL 8 /* code uses literals */
#define SEEN_FUNC 16 /* calls C functions */
#define SEEN_TAIL_CALL 32 /* code uses tail calls */
#define SEEN_STACK (SEEN_FUNC | SEEN_MEM | SEEN_SKB)

/*
Expand All @@ -60,6 +64,7 @@ struct bpf_jit {
#define REG_L (__MAX_BPF_REG+3) /* Literal pool register */
#define REG_15 (__MAX_BPF_REG+4) /* Register 15 */
#define REG_0 REG_W0 /* Register 0 */
#define REG_1 REG_W1 /* Register 1 */
#define REG_2 BPF_REG_1 /* Register 2 */
#define REG_14 BPF_REG_0 /* Register 14 */

Expand Down Expand Up @@ -223,6 +228,24 @@ static inline void reg_set_seen(struct bpf_jit *jit, u32 b1)
REG_SET_SEEN(b3); \
})

#define EMIT6_PCREL_LABEL(op1, op2, b1, b2, label, mask) \
({ \
int rel = (jit->labels[label] - jit->prg) >> 1; \
_EMIT6(op1 | reg(b1, b2) << 16 | (rel & 0xffff), \
op2 | mask << 12); \
REG_SET_SEEN(b1); \
REG_SET_SEEN(b2); \
})

#define EMIT6_PCREL_IMM_LABEL(op1, op2, b1, imm, label, mask) \
({ \
int rel = (jit->labels[label] - jit->prg) >> 1; \
_EMIT6(op1 | (reg_high(b1) | mask) << 16 | \
(rel & 0xffff), op2 | (imm & 0xff) << 8); \
REG_SET_SEEN(b1); \
BUILD_BUG_ON(((unsigned long) imm) > 0xff); \
})

#define EMIT6_PCREL(op1, op2, b1, b2, i, off, mask) \
({ \
/* Branch instruction needs 6 bytes */ \
Expand Down Expand Up @@ -286,7 +309,7 @@ static void jit_fill_hole(void *area, unsigned int size)
*/
static void save_regs(struct bpf_jit *jit, u32 rs, u32 re)
{
u32 off = 72 + (rs - 6) * 8;
u32 off = STK_OFF_R6 + (rs - 6) * 8;

if (rs == re)
/* stg %rs,off(%r15) */
Expand All @@ -301,7 +324,7 @@ static void save_regs(struct bpf_jit *jit, u32 rs, u32 re)
*/
static void restore_regs(struct bpf_jit *jit, u32 rs, u32 re)
{
u32 off = 72 + (rs - 6) * 8;
u32 off = STK_OFF_R6 + (rs - 6) * 8;

if (jit->seen & SEEN_STACK)
off += STK_OFF;
Expand Down Expand Up @@ -374,6 +397,16 @@ static void save_restore_regs(struct bpf_jit *jit, int op)
*/
static void bpf_jit_prologue(struct bpf_jit *jit)
{
if (jit->seen & SEEN_TAIL_CALL) {
/* xc STK_OFF_TCCNT(4,%r15),STK_OFF_TCCNT(%r15) */
_EMIT6(0xd703f000 | STK_OFF_TCCNT, 0xf000 | STK_OFF_TCCNT);
} else {
/* j tail_call_start: NOP if no tail calls are used */
EMIT4_PCREL(0xa7f40000, 6);
_EMIT2(0);
}
/* Tail calls have to skip above initialization */
jit->tail_call_start = jit->prg;
/* Save registers */
save_restore_regs(jit, REGS_SAVE);
/* Setup literal pool */
Expand Down Expand Up @@ -951,6 +984,75 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp, int i
EMIT4(0xb9040000, BPF_REG_0, REG_2);
break;
}
case BPF_JMP | BPF_CALL | BPF_X:
/*
* Implicit input:
* B1: pointer to ctx
* B2: pointer to bpf_array
* B3: index in bpf_array
*/
jit->seen |= SEEN_TAIL_CALL;

/*
* if (index >= array->map.max_entries)
* goto out;
*/

/* llgf %w1,map.max_entries(%b2) */
EMIT6_DISP_LH(0xe3000000, 0x0016, REG_W1, REG_0, BPF_REG_2,
offsetof(struct bpf_array, map.max_entries));
/* clgrj %b3,%w1,0xa,label0: if %b3 >= %w1 goto out */
EMIT6_PCREL_LABEL(0xec000000, 0x0065, BPF_REG_3,
REG_W1, 0, 0xa);

/*
* if (tail_call_cnt++ > MAX_TAIL_CALL_CNT)
* goto out;
*/

if (jit->seen & SEEN_STACK)
off = STK_OFF_TCCNT + STK_OFF;
else
off = STK_OFF_TCCNT;
/* lhi %w0,1 */
EMIT4_IMM(0xa7080000, REG_W0, 1);
/* laal %w1,%w0,off(%r15) */
EMIT6_DISP_LH(0xeb000000, 0x00fa, REG_W1, REG_W0, REG_15, off);
/* clij %w1,MAX_TAIL_CALL_CNT,0x2,label0 */
EMIT6_PCREL_IMM_LABEL(0xec000000, 0x007f, REG_W1,
MAX_TAIL_CALL_CNT, 0, 0x2);

/*
* prog = array->prog[index];
* if (prog == NULL)
* goto out;
*/

/* sllg %r1,%b3,3: %r1 = index * 8 */
EMIT6_DISP_LH(0xeb000000, 0x000d, REG_1, BPF_REG_3, REG_0, 3);
/* lg %r1,prog(%b2,%r1) */
EMIT6_DISP_LH(0xe3000000, 0x0004, REG_1, BPF_REG_2,
REG_1, offsetof(struct bpf_array, prog));
/* clgij %r1,0,0x8,label0 */
EMIT6_PCREL_IMM_LABEL(0xec000000, 0x007d, REG_1, 0, 0, 0x8);

/*
* Restore registers before calling function
*/
save_restore_regs(jit, REGS_RESTORE);

/*
* goto *(prog->bpf_func + tail_call_start);
*/

/* lg %r1,bpf_func(%r1) */
EMIT6_DISP_LH(0xe3000000, 0x0004, REG_1, REG_1, REG_0,
offsetof(struct bpf_prog, bpf_func));
/* bc 0xf,tail_call_start(%r1) */
_EMIT4(0x47f01000 + jit->tail_call_start);
/* out: */
jit->labels[0] = jit->prg;
break;
case BPF_JMP | BPF_EXIT: /* return b0 */
last = (i == fp->len - 1) ? 1 : 0;
if (last && !(jit->seen & SEEN_RET0))
Expand Down

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