[Very RFC PATCH 2/3] insn: Add arch64_insn_gen_branch_imm to generate branch

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

We copied from Linux code (v4.7) the code that generates
the unconditional branch instructions. This is mostly
from Linux commit c0cafbae20d2878883ec3c06d6ea30ff38a6bf92
"arm64: introduce aarch64_insn_gen_branch_reg()"

However the branch-link instructions was omitted, only
the branch instruction is generated.

This lays the groundwork for Livepatch to generate the
trampoline to jump to the new replacement function.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
--
Cc: Ross Lagerwall <ross.lagerwall@citrix.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Julien Grall <julien.grall@arm.com>
---
 xen/arch/arm/arm64/insn.c        | 41 ++++++++++++++++++++++++++++++++++++++++
 xen/include/asm-arm/arm64/insn.h |  2 ++
 2 files changed, 43 insertions(+)

diff --git a/xen/arch/arm/arm64/insn.c b/xen/arch/arm/arm64/insn.c
index 12b4d96..15dd7bc 100644
--- a/xen/arch/arm/arm64/insn.c
+++ b/xen/arch/arm/arm64/insn.c
@@ -209,6 +209,47 @@ u32 aarch64_set_branch_offset(u32 insn, s32 offset)
 	BUG();
 }
 
+static inline long branch_imm_common(unsigned long pc, unsigned long addr,
+				     long range)
+{
+	long offset;
+
+	if ((pc & 0x3) || (addr & 0x3)) {
+		pr_err("%s: A64 instructions must be word aligned\n", __func__);
+		return range;
+	}
+
+	offset = ((long)addr - (long)pc);
+
+	if (offset < -range || offset >= range) {
+		pr_err("%s: offset out of range\n", __func__);
+		return range;
+	}
+
+	return offset;
+}
+
+u32 __kprobes aarch64_insn_gen_branch_imm(unsigned long pc, unsigned long addr)
+{
+	u32 insn;
+	long offset;
+
+	/*
+	 * B/BL support [-128M, 128M) offset
+	 * ARM64 virtual address arrangement guarantees all kernel and module
+	 * texts are within +/-128M.
+	 */
+	offset = branch_imm_common(pc, addr, SZ_128M);
+	if (offset >= SZ_128M)
+		return AARCH64_BREAK_FAULT;
+
+	insn = aarch64_insn_get_b_value();
+
+	return aarch64_insn_encode_immediate(AARCH64_INSN_IMM_26, insn,
+					     offset >> 2);
+}
+
+
 /*
  * Local variables:
  * mode: C
diff --git a/xen/include/asm-arm/arm64/insn.h b/xen/include/asm-arm/arm64/insn.h
index 6ce37be..378ba11 100644
--- a/xen/include/asm-arm/arm64/insn.h
+++ b/xen/include/asm-arm/arm64/insn.h
@@ -61,6 +61,8 @@ u32 aarch64_insn_encode_immediate(enum aarch64_insn_imm_type type,
 s32 aarch64_get_branch_offset(u32 insn);
 u32 aarch64_set_branch_offset(u32 insn, s32 offset);
 
+u32 aarch64_insn_gen_branch_imm(unsigned long pc, unsigned long addr);
+
 /* Wrapper for common code */
 static inline bool insn_is_branch_imm(u32 insn)
 {
-- 
2.4.11


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