powerpc/bpf: Reallocate BPF registers to volatile registers when possible on PPC32

When the BPF routine doesn't call any function, the non volatile
registers can be reallocated to volatile registers in order to
avoid having to save them/restore on the stack.

Before this patch, the test #359 ADD default X is:

   0:	7c 64 1b 78 	mr      r4,r3
   4:	38 60 00 00 	li      r3,0
   8:	94 21 ff b0 	stwu    r1,-80(r1)
   c:	60 00 00 00 	nop
  10:	92 e1 00 2c 	stw     r23,44(r1)
  14:	93 01 00 30 	stw     r24,48(r1)
  18:	93 21 00 34 	stw     r25,52(r1)
  1c:	93 41 00 38 	stw     r26,56(r1)
  20:	39 80 00 00 	li      r12,0
  24:	39 60 00 00 	li      r11,0
  28:	3b 40 00 00 	li      r26,0
  2c:	3b 20 00 00 	li      r25,0
  30:	7c 98 23 78 	mr      r24,r4
  34:	7c 77 1b 78 	mr      r23,r3
  38:	39 80 00 42 	li      r12,66
  3c:	39 60 00 00 	li      r11,0
  40:	7d 8c d2 14 	add     r12,r12,r26
  44:	39 60 00 00 	li      r11,0
  48:	7d 83 63 78 	mr      r3,r12
  4c:	82 e1 00 2c 	lwz     r23,44(r1)
  50:	83 01 00 30 	lwz     r24,48(r1)
  54:	83 21 00 34 	lwz     r25,52(r1)
  58:	83 41 00 38 	lwz     r26,56(r1)
  5c:	38 21 00 50 	addi    r1,r1,80
  60:	4e 80 00 20 	blr

After this patch, the same test has become:

   0:	7c 64 1b 78 	mr      r4,r3
   4:	38 60 00 00 	li      r3,0
   8:	94 21 ff b0 	stwu    r1,-80(r1)
   c:	60 00 00 00 	nop
  10:	39 80 00 00 	li      r12,0
  14:	39 60 00 00 	li      r11,0
  18:	39 00 00 00 	li      r8,0
  1c:	38 e0 00 00 	li      r7,0
  20:	7c 86 23 78 	mr      r6,r4
  24:	7c 65 1b 78 	mr      r5,r3
  28:	39 80 00 42 	li      r12,66
  2c:	39 60 00 00 	li      r11,0
  30:	7d 8c 42 14 	add     r12,r12,r8
  34:	39 60 00 00 	li      r11,0
  38:	7d 83 63 78 	mr      r3,r12
  3c:	38 21 00 50 	addi    r1,r1,80
  40:	4e 80 00 20 	blr
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/b94562d7d2bb21aec89de0c40bb3cd91054b65a2.1616430991.git.christophe.leroy@csgroup.eu

arch/powerpc/net/bpf_jit.h

@@ -116,6 +116,15 @@ static inline bool is_nearbranch(int offset)
 #define SEEN_STACK	0x40000000 /* uses BPF stack */
 #define SEEN_TAILCALL	0x80000000 /* uses tail calls */
 
+#define SEEN_VREG_MASK	0x1ff80000 /* Volatile registers r3-r12 */
+#define SEEN_NVREG_MASK	0x0003ffff /* Non volatile registers r14-r31 */
+
+#ifdef CONFIG_PPC64
+extern const int b2p[MAX_BPF_JIT_REG + 2];
+#else
+extern const int b2p[MAX_BPF_JIT_REG + 1];
+#endif
+
 struct codegen_context {
 	/*
 	 * This is used to track register usage as well
@@ -129,6 +138,7 @@ struct codegen_context {
 	unsigned int seen;
 	unsigned int idx;
 	unsigned int stack_size;
+	int b2p[ARRAY_SIZE(b2p)];
 };
 
 static inline void bpf_flush_icache(void *start, void *end)
@@ -147,11 +157,17 @@ static inline void bpf_set_seen_register(struct codegen_context *ctx, int i)
 	ctx->seen |= 1 << (31 - i);
 }
 
+static inline void bpf_clear_seen_register(struct codegen_context *ctx, int i)
+{
+	ctx->seen &= ~(1 << (31 - i));
+}
+
 void bpf_jit_emit_func_call_rel(u32 *image, struct codegen_context *ctx, u64 func);
 int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *ctx,
 		       u32 *addrs, bool extra_pass);
 void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx);
 void bpf_jit_build_epilogue(u32 *image, struct codegen_context *ctx);
+void bpf_jit_realloc_regs(struct codegen_context *ctx);
 
 #endif
 

arch/powerpc/net/bpf_jit64.h

@@ -39,7 +39,7 @@
 #define TMP_REG_2	(MAX_BPF_JIT_REG + 1)
 
 /* BPF to ppc register mappings */
-static const int b2p[] = {
+const int b2p[MAX_BPF_JIT_REG + 2] = {
 	/* function return value */
 	[BPF_REG_0] = 8,
 	/* function arguments */

arch/powerpc/net/bpf_jit_comp.c

@@ -143,6 +143,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
 	}
 
 	memset(&cgctx, 0, sizeof(struct codegen_context));
+	memcpy(cgctx.b2p, b2p, sizeof(cgctx.b2p));
 
 	/* Make sure that the stack is quadword aligned. */
 	cgctx.stack_size = round_up(fp->aux->stack_depth, 16);
@@ -167,6 +168,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
 		}
 	}
 
+	bpf_jit_realloc_regs(&cgctx);
 	/*
 	 * Pretend to build prologue, given the features we've seen.  This will
 	 * update ctgtx.idx as it pretends to output instructions, then we can

arch/powerpc/net/bpf_jit_comp32.c

@@ -37,7 +37,7 @@
 #define TMP_REG	(MAX_BPF_JIT_REG + 0)
 
 /* BPF to ppc register mappings */
-static const int b2p[] = {
+const int b2p[MAX_BPF_JIT_REG + 1] = {
 	/* function return value */
 	[BPF_REG_0] = 12,
 	/* function arguments */
@@ -60,7 +60,7 @@ static const int b2p[] = {
 
 static int bpf_to_ppc(struct codegen_context *ctx, int reg)
 {
-	return b2p[reg];
+	return ctx->b2p[reg];
 }
 
 /* PPC NVR range -- update this if we ever use NVRs below r17 */
@@ -77,6 +77,32 @@ static int bpf_jit_stack_offsetof(struct codegen_context *ctx, int reg)
 	return BPF_PPC_STACKFRAME(ctx) - 4;
 }
 
+void bpf_jit_realloc_regs(struct codegen_context *ctx)
+{
+	if (ctx->seen & SEEN_FUNC)
+		return;
+
+	while (ctx->seen & SEEN_NVREG_MASK &&
+	      (ctx->seen & SEEN_VREG_MASK) != SEEN_VREG_MASK) {
+		int old = 32 - fls(ctx->seen & (SEEN_NVREG_MASK & 0xaaaaaaab));
+		int new = 32 - fls(~ctx->seen & (SEEN_VREG_MASK & 0xaaaaaaaa));
+		int i;
+
+		for (i = BPF_REG_0; i <= TMP_REG; i++) {
+			if (ctx->b2p[i] != old)
+				continue;
+			ctx->b2p[i] = new;
+			bpf_set_seen_register(ctx, new);
+			bpf_clear_seen_register(ctx, old);
+			if (i != TMP_REG) {
+				bpf_set_seen_register(ctx, new - 1);
+				bpf_clear_seen_register(ctx, old - 1);
+			}
+			break;
+		}
+	}
+}
+
 void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
 {
 	int i;

arch/powerpc/net/bpf_jit_comp64.c

@@ -64,6 +64,10 @@ static int bpf_jit_stack_offsetof(struct codegen_context *ctx, int reg)
 	BUG();
 }
 
+void bpf_jit_realloc_regs(struct codegen_context *ctx)
+{
+}
+
 void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
 {
 	int i;