BACKPORT: RISC-V: Stop relying on GCC's register allocator's hueristics
GCC allows users to hint to the register allocation that a variable should be
placed in a register by using a syntax along the lines of
function(...) {
register long in_REG __asm__("REG");
}
We've abused this a bit throughout the RISC-V port to access fixed registers
directly as C variables. In practice it's never going to blow up because GCC
isn't going to allocate these registers, but it's not a well defined syntax so
we really shouldn't be relying upon this. Luckily there is a very similar but
well defined syntax that allows us to still access these registers directly as
C variables, which is to simply declare the register variables globally. For
fixed variables this doesn't change the ABI.
LLVM disallows this ambiguous syntax, so this isn't just strictly a formatting
change.
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
(cherry picked from commit 52e7c52d2ded5908e6a4f8a7248e5fa6e0d6809a)
[tkjos: minor conflict resolution in arch/riscv/kernel/process.c]
Change-Id: Ie4f7852fc12c6e6c368948f17e8649b8a0447030
diff --git a/arch/riscv/include/asm/current.h b/arch/riscv/include/asm/current.h
index 44dcf7f..f3d89c4 100644
--- a/arch/riscv/include/asm/current.h
+++ b/arch/riscv/include/asm/current.h
@@ -17,6 +17,8 @@
struct task_struct;
+register struct task_struct *riscv_current_is_tp __asm__("tp");
+
/*
* This only works because "struct thread_info" is at offset 0 from "struct
* task_struct". This constraint seems to be necessary on other architectures
@@ -26,8 +28,7 @@ struct task_struct;
*/
static __always_inline struct task_struct *get_current(void)
{
- register struct task_struct *tp __asm__("tp");
- return tp;
+ return riscv_current_is_tp;
}
#define current get_current()
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index 9d4b409..652ba81 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -22,6 +22,8 @@
#include <asm/switch_to.h>
#include <asm/thread_info.h>
+unsigned long gp_in_global __asm__("gp");
+
extern asmlinkage void ret_from_fork(void);
extern asmlinkage void ret_from_kernel_thread(void);
@@ -109,9 +111,8 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long usp,
/* p->thread holds context to be restored by __switch_to() */
if (unlikely(p->flags & PF_KTHREAD)) {
/* Kernel thread */
- const register unsigned long gp __asm__ ("gp");
memset(childregs, 0, sizeof(struct pt_regs));
- childregs->gp = gp;
+ childregs->gp = gp_in_global;
childregs->sstatus = SR_SPP | SR_SPIE; /* Supervisor, irqs on */
p->thread.ra = (unsigned long)ret_from_kernel_thread;
diff --git a/arch/riscv/kernel/stacktrace.c b/arch/riscv/kernel/stacktrace.c
index 5ba4d23..7d145ae7 100644
--- a/arch/riscv/kernel/stacktrace.c
+++ b/arch/riscv/kernel/stacktrace.c
@@ -19,6 +19,8 @@ struct stackframe {
unsigned long ra;
};
+register unsigned long sp_in_global __asm__("sp");
+
void notrace walk_stackframe(struct task_struct *task, struct pt_regs *regs,
bool (*fn)(unsigned long, void *), void *arg)
{
@@ -29,7 +31,7 @@ void notrace walk_stackframe(struct task_struct *task, struct pt_regs *regs,
sp = user_stack_pointer(regs);
pc = instruction_pointer(regs);
} else if (task == NULL || task == current) {
- const register unsigned long current_sp __asm__ ("sp");
+ const register unsigned long current_sp = sp_in_global;
fp = (unsigned long)__builtin_frame_address(0);
sp = current_sp;
pc = (unsigned long)walk_stackframe;
@@ -73,8 +75,7 @@ void notrace walk_stackframe(struct task_struct *task,
sp = user_stack_pointer(regs);
pc = instruction_pointer(regs);
} else if (task == NULL || task == current) {
- const register unsigned long current_sp __asm__ ("sp");
- sp = current_sp;
+ sp = sp_in_global;
pc = (unsigned long)walk_stackframe;
} else {
/* task blocked in __switch_to */
