unimore/kernel-hacking-2024-linux-steffo
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RISC-V: Stop relying on GCC's register allocator's hueristics

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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>
This commit is contained in:
Palmer Dabbelt 2020-02-27 11:07:28 -08:00
parent 064223b947
commit 52e7c52d2d
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GPG key ID: 2E1319F35FBB1889
3 changed files with 10 additions and 7 deletions
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5
arch/riscv/include/asm/current.h
View file

@ -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()

5
arch/riscv/kernel/process.c
View file

@ -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);
@ -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;

7
arch/riscv/kernel/stacktrace.c
View file

@ -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 */