From 52e7c52d2ded5908e6a4f8a7248e5fa6e0d6809a Mon Sep 17 00:00:00 2001 From: Palmer Dabbelt Date: Thu, 27 Feb 2020 11:07:28 -0800 Subject: [PATCH] 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 --- arch/riscv/include/asm/current.h | 5 +++-- arch/riscv/kernel/process.c | 5 +++-- arch/riscv/kernel/stacktrace.c | 7 ++++--- 3 files changed, 10 insertions(+), 7 deletions(-) diff --git a/arch/riscv/include/asm/current.h b/arch/riscv/include/asm/current.h index dd973efe5d7c..1de233d8e8de 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 817cf7b0974c..610c11e91606 100644 --- a/arch/riscv/kernel/process.c +++ b/arch/riscv/kernel/process.c @@ -22,6 +22,8 @@ #include #include +unsigned long gp_in_global __asm__("gp"); + extern asmlinkage void ret_from_fork(void); extern asmlinkage void ret_from_kernel_thread(void); @@ -107,9 +109,8 @@ 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; /* Supervisor/Machine, irqs on: */ childregs->status = SR_PP | SR_PIE; diff --git a/arch/riscv/kernel/stacktrace.c b/arch/riscv/kernel/stacktrace.c index 0940681d2f68..02087fe539c6 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 @@ static 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 */