s390/bpf: Align literal pool entries

When literal pool size exceeds 512k, it's no longer possible to
reference all the entries in it using a single base register and long
displacement. Therefore, PC-relative lgfrl and lgrl instructions need to
be used.

Unfortunately, they require their arguments to be aligned to 4- and
8-byte boundaries respectively. This generates certain overhead due to
necessary padding bytes. Grouping 4- and 8-byte entries together reduces
the maximum overhead to 6 bytes (2 for aligning 4-byte entries and 4 for
aligning 8-byte entries).

While in theory it is possible to detect whether or not alignment is
needed by comparing the literal pool size with 512k, in practice this
leads to having two ways of emitting constants, making the code more
complicated.

Prefer code simplicity over trivial size saving, and always group and
align literal pool entries.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191118180340.68373-3-iii@linux.ibm.com

arch/s390/net/bpf_jit_comp.c

@@ -24,6 +24,7 @@
 #include <linux/init.h>
 #include <linux/bpf.h>
 #include <linux/mm.h>
+#include <linux/kernel.h>
 #include <asm/cacheflush.h>
 #include <asm/dis.h>
 #include <asm/facility.h>
@@ -39,8 +40,10 @@ struct bpf_jit {
 	int size;		/* Size of program and literal pool */
 	int size_prg;		/* Size of program */
 	int prg;		/* Current position in program */
-	int lit_start;		/* Start of literal pool */
-	int lit;		/* Current position in literal pool */
+	int lit32_start;	/* Start of 32-bit literal pool */
+	int lit32;		/* Current position in 32-bit literal pool */
+	int lit64_start;	/* Start of 64-bit literal pool */
+	int lit64;		/* Current position in 64-bit literal pool */
 	int base_ip;		/* Base address for literal pool */
 	int exit_ip;		/* Address of exit */
 	int r1_thunk_ip;	/* Address of expoline thunk for 'br %r1' */
@@ -287,22 +290,22 @@ static inline void reg_set_seen(struct bpf_jit *jit, u32 b1)
 #define EMIT_CONST_U32(val)					\
 ({								\
 	unsigned int ret;					\
-	ret = jit->lit - jit->base_ip;				\
+	ret = jit->lit32 - jit->base_ip;			\
 	jit->seen |= SEEN_LITERAL;				\
 	if (jit->prg_buf)					\
-		*(u32 *) (jit->prg_buf + jit->lit) = (u32) (val);\
-	jit->lit += 4;						\
+		*(u32 *)(jit->prg_buf + jit->lit32) = (u32)(val);\
+	jit->lit32 += 4;					\
 	ret;							\
 })
 
 #define EMIT_CONST_U64(val)					\
 ({								\
 	unsigned int ret;					\
-	ret = jit->lit - jit->base_ip;				\
+	ret = jit->lit64 - jit->base_ip;			\
 	jit->seen |= SEEN_LITERAL;				\
 	if (jit->prg_buf)					\
-		*(u64 *) (jit->prg_buf + jit->lit) = (u64) (val);\
-	jit->lit += 8;						\
+		*(u64 *)(jit->prg_buf + jit->lit64) = (u64)(val);\
+	jit->lit64 += 8;					\
 	ret;							\
 })
 
@@ -1430,9 +1433,10 @@ static int bpf_set_addr(struct bpf_jit *jit, int i)
 static int bpf_jit_prog(struct bpf_jit *jit, struct bpf_prog *fp,
 			bool extra_pass)
 {
-	int i, insn_count;
+	int i, insn_count, lit32_size, lit64_size;
 
-	jit->lit = jit->lit_start;
+	jit->lit32 = jit->lit32_start;
+	jit->lit64 = jit->lit64_start;
 	jit->prg = 0;
 
 	bpf_jit_prologue(jit, fp->aux->stack_depth);
@@ -1448,8 +1452,15 @@ static int bpf_jit_prog(struct bpf_jit *jit, struct bpf_prog *fp,
 	}
 	bpf_jit_epilogue(jit, fp->aux->stack_depth);
 
-	jit->lit_start = jit->prg;
-	jit->size = jit->lit;
+	lit32_size = jit->lit32 - jit->lit32_start;
+	lit64_size = jit->lit64 - jit->lit64_start;
+	jit->lit32_start = jit->prg;
+	if (lit32_size)
+		jit->lit32_start = ALIGN(jit->lit32_start, 4);
+	jit->lit64_start = jit->lit32_start + lit32_size;
+	if (lit64_size)
+		jit->lit64_start = ALIGN(jit->lit64_start, 8);
+	jit->size = jit->lit64_start + lit64_size;
 	jit->size_prg = jit->prg;
 	return 0;
 }
@@ -1535,7 +1546,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
 		goto free_addrs;
 	}
 
-	header = bpf_jit_binary_alloc(jit.size, &jit.prg_buf, 2, jit_fill_hole);
+	header = bpf_jit_binary_alloc(jit.size, &jit.prg_buf, 8, jit_fill_hole);
 	if (!header) {
 		fp = orig_fp;
 		goto free_addrs;