Since all immediate branches are PC-relative on Aarch64, these
instructions cannot be used as an alternative with the simplistic
approach we currently have (the immediate has been computed from
the .altinstr_replacement section, and end-up being completely off
if we insert it directly).
This patch handles the b and bl instructions in a different way,
using the insn framework to recompute the immediate, and generate
the right displacement.
Reviewed-by: Andre Przywara <andre.przyw...@arm.com>
Acked-by: Will Deacon <will.dea...@arm.com>
Signed-off-by: Marc Zyngier <marc.zyng...@arm.com>
---
arch/arm64/kernel/alternative.c | 55 +++++++++++++++++++++++++++++++++++++++--
1 file changed, 53 insertions(+), 2 deletions(-)
diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
index ad7821d..21033bb 100644
--- a/arch/arm64/kernel/alternative.c
+++ b/arch/arm64/kernel/alternative.c
@@ -24,6 +24,7 @@
#include <asm/cacheflush.h>
#include <asm/alternative.h>
#include <asm/cpufeature.h>
+#include <asm/insn.h>
#include <linux/stop_machine.h>
extern struct alt_instr __alt_instructions[], __alt_instructions_end[];
@@ -33,6 +34,48 @@ struct alt_region {
struct alt_instr *end;
};
+/*
+ * Decode the imm field of a b/bl instruction, and return the byte
+ * offset as a signed value (so it can be used when computing a new
+ * branch target).
+ */
+static s32 get_branch_offset(u32 insn)
+{
+ s32 imm = aarch64_insn_decode_immediate(AARCH64_INSN_IMM_26, insn);
+
+ /* sign-extend the immediate before turning it into a byte offset */
+ return (imm << 6) >> 4;
+}
+
+static u32 get_alt_insn(u8 *insnptr, u8 *altinsnptr)
+{
+ u32 insn;
+
+ aarch64_insn_read(altinsnptr, &insn);
+
+ /* Stop the world on instructions we don't support... */
+ BUG_ON(aarch64_insn_is_cbz(insn));
+ BUG_ON(aarch64_insn_is_cbnz(insn));
+ BUG_ON(aarch64_insn_is_bcond(insn));
+ /* ... and there is probably more. */
+
+ if (aarch64_insn_is_b(insn) || aarch64_insn_is_bl(insn)) {
+ enum aarch64_insn_branch_type type;
+ unsigned long target;
+
+ if (aarch64_insn_is_b(insn))
+ type = AARCH64_INSN_BRANCH_NOLINK;
+ else
+ type = AARCH64_INSN_BRANCH_LINK;
+
+ target = (unsigned long)altinsnptr + get_branch_offset(insn);
+ insn = aarch64_insn_gen_branch_imm((unsigned long)insnptr,
+ target, type);
+ }
+
+ return insn;
+}
+
static int __apply_alternatives(void *alt_region)
{
struct alt_instr *alt;
@@ -40,16 +83,24 @@ static int __apply_alternatives(void *alt_region)
u8 *origptr, *replptr;
for (alt = region->begin; alt < region->end; alt++) {
+ u32 insn;
+ int i;
+
if (!cpus_have_cap(alt->cpufeature))
continue;
- BUG_ON(alt->alt_len > alt->orig_len);
+ BUG_ON(alt->alt_len != alt->orig_len);
pr_info_once("patching kernel code\n");
origptr = (u8 *)&alt->orig_offset + alt->orig_offset;
replptr = (u8 *)&alt->alt_offset + alt->alt_offset;
- memcpy(origptr, replptr, alt->alt_len);
+
+ for (i = 0; i < alt->alt_len; i += sizeof(insn)) {
+ insn = get_alt_insn(origptr + i, replptr + i);
+ aarch64_insn_write(origptr + i, insn);
+ }
+
flush_icache_range((uintptr_t)origptr,
(uintptr_t)(origptr + alt->alt_len));
}
--
2.1.4
_______________________________________________
kvmarm mailing list
kvmarm@lists.cs.columbia.edu
https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
