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* [PATCH bpf-next] bpf,x64: Save bytes for DIV by reducing reg copies
@ 2021-09-29 23:47 Jie Meng
  2021-09-30 22:50 ` Daniel Borkmann
  0 siblings, 1 reply; 2+ messages in thread
From: Jie Meng @ 2021-09-29 23:47 UTC (permalink / raw)
  To: bpf, ast, andrii, daniel; +Cc: Jie Meng

Instead of unconditionally performing push/pop on rax/rdx in case of
division/modulo, we can save a few bytes in case of dest register
being either BPF r0 (rax) or r3 (rdx) since the result is written in
there anyway.

Also, we do not need to copy src to r11 unless src is either rax, rdx
or an immediate.

Signed-off-by: Jie Meng <jmeng@fb.com>
---
 arch/x86/net/bpf_jit_comp.c                | 71 +++++++++++++---------
 tools/testing/selftests/bpf/verifier/jit.c | 28 +++++++++
 2 files changed, 70 insertions(+), 29 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 20d2d6a1f9de..346b4131d496 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1028,19 +1028,30 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
 		case BPF_ALU64 | BPF_MOD | BPF_X:
 		case BPF_ALU64 | BPF_DIV | BPF_X:
 		case BPF_ALU64 | BPF_MOD | BPF_K:
-		case BPF_ALU64 | BPF_DIV | BPF_K:
-			EMIT1(0x50); /* push rax */
-			EMIT1(0x52); /* push rdx */
-
-			if (BPF_SRC(insn->code) == BPF_X)
-				/* mov r11, src_reg */
-				EMIT_mov(AUX_REG, src_reg);
-			else
+		case BPF_ALU64 | BPF_DIV | BPF_K: {
+			bool is64 = BPF_CLASS(insn->code) == BPF_ALU64;
+
+			if (dst_reg != BPF_REG_0)
+				EMIT1(0x50); /* push rax */
+			if (dst_reg != BPF_REG_3)
+				EMIT1(0x52); /* push rdx */
+
+			if (BPF_SRC(insn->code) == BPF_X) {
+				if (src_reg == BPF_REG_0 ||
+				    src_reg == BPF_REG_3) {
+					/* mov r11, src_reg */
+					EMIT_mov(AUX_REG, src_reg);
+					src_reg = AUX_REG;
+				}
+			} else {
 				/* mov r11, imm32 */
 				EMIT3_off32(0x49, 0xC7, 0xC3, imm32);
+				src_reg = AUX_REG;
+			}
 
-			/* mov rax, dst_reg */
-			EMIT_mov(BPF_REG_0, dst_reg);
+			if (dst_reg != BPF_REG_0)
+				/* mov rax, dst_reg */
+				emit_mov_reg(&prog, is64, BPF_REG_0, dst_reg);
 
 			/*
 			 * xor edx, edx
@@ -1048,26 +1059,28 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
 			 */
 			EMIT2(0x31, 0xd2);
 
-			if (BPF_CLASS(insn->code) == BPF_ALU64)
-				/* div r11 */
-				EMIT3(0x49, 0xF7, 0xF3);
-			else
-				/* div r11d */
-				EMIT3(0x41, 0xF7, 0xF3);
-
-			if (BPF_OP(insn->code) == BPF_MOD)
-				/* mov r11, rdx */
-				EMIT3(0x49, 0x89, 0xD3);
-			else
-				/* mov r11, rax */
-				EMIT3(0x49, 0x89, 0xC3);
-
-			EMIT1(0x5A); /* pop rdx */
-			EMIT1(0x58); /* pop rax */
-
-			/* mov dst_reg, r11 */
-			EMIT_mov(dst_reg, AUX_REG);
+			if (is64)
+				EMIT1(add_1mod(0x48, src_reg));
+			else if (is_ereg(src_reg))
+				EMIT1(add_1mod(0x40, src_reg));
+			/* div src_reg */
+			EMIT2(0xF7, add_1reg(0xF0, src_reg));
+
+			if (BPF_OP(insn->code) == BPF_MOD &&
+			    dst_reg != BPF_REG_3)
+				/* mov dst_reg, rdx */
+				emit_mov_reg(&prog, is64, dst_reg, BPF_REG_3);
+			else if (BPF_OP(insn->code) == BPF_DIV &&
+				 dst_reg != BPF_REG_0)
+				/* mov dst_reg, rax */
+				emit_mov_reg(&prog, is64, dst_reg, BPF_REG_0);
+
+			if (dst_reg != BPF_REG_3)
+				EMIT1(0x5A); /* pop rdx */
+			if (dst_reg != BPF_REG_0)
+				EMIT1(0x58); /* pop rax */
 			break;
+		}
 
 		case BPF_ALU | BPF_MUL | BPF_K:
 		case BPF_ALU64 | BPF_MUL | BPF_K:
diff --git a/tools/testing/selftests/bpf/verifier/jit.c b/tools/testing/selftests/bpf/verifier/jit.c
index eedcb752bf70..0f2583f0685a 100644
--- a/tools/testing/selftests/bpf/verifier/jit.c
+++ b/tools/testing/selftests/bpf/verifier/jit.c
@@ -102,6 +102,34 @@
 	.result = ACCEPT,
 	.retval = 2,
 },
+{
+	"jit: various div tests",
+	.insns = {
+	BPF_LD_IMM64(BPF_REG_2, 0xefeffeULL),
+	BPF_LD_IMM64(BPF_REG_0, 0xeeff0d413122ULL),
+	BPF_LD_IMM64(BPF_REG_1, 0xfefeeeULL),
+	BPF_ALU64_REG(BPF_DIV, BPF_REG_0, BPF_REG_1),
+	BPF_JMP_REG(BPF_JEQ, BPF_REG_0, BPF_REG_2, 2),
+	BPF_MOV64_IMM(BPF_REG_0, 1),
+	BPF_EXIT_INSN(),
+	BPF_LD_IMM64(BPF_REG_2, 0xaa93ULL),
+	BPF_ALU64_IMM(BPF_MOD, BPF_REG_1, 0xbeefULL),
+	BPF_JMP_REG(BPF_JEQ, BPF_REG_1, BPF_REG_2, 2),
+	BPF_MOV64_IMM(BPF_REG_0, 1),
+	BPF_EXIT_INSN(),
+	BPF_LD_IMM64(BPF_REG_2, 0x5ee1dULL),
+	BPF_LD_IMM64(BPF_REG_1, 0xfefeeeULL),
+	BPF_LD_IMM64(BPF_REG_3, 0x2bULL),
+	BPF_ALU32_REG(BPF_DIV, BPF_REG_1, BPF_REG_3),
+	BPF_JMP_REG(BPF_JEQ, BPF_REG_1, BPF_REG_2, 2),
+	BPF_MOV64_IMM(BPF_REG_0, 1),
+	BPF_EXIT_INSN(),
+	BPF_MOV64_IMM(BPF_REG_0, 2),
+	BPF_EXIT_INSN(),
+	},
+	.result = ACCEPT,
+	.retval = 2,
+},
 {
 	"jit: jsgt, jslt",
 	.insns = {
-- 
2.30.2


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2021-09-29 23:47 [PATCH bpf-next] bpf,x64: Save bytes for DIV by reducing reg copies Jie Meng
2021-09-30 22:50 ` Daniel Borkmann

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