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riscv: Correct the initialized flow of FP register

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The following two reasons cause FP registers are sometimes not
initialized before starting the user program.
1. Currently, the FP context is initialized in flush_thread() function
   and we expect these initial values to be restored to FP register when
   doing FP context switch. However, the FP context switch only occurs in
   switch_to function. Hence, if this process does not be scheduled out
   and scheduled in before entering the user space, the FP registers
   have no chance to initialize.
2. In flush_thread(), the state of reg->sstatus.FS inherits from the
   parent. Hence, the state of reg->sstatus.FS may be dirty. If this
   process is scheduled out during flush_thread() and initializing the
   FP register, the fstate_save() in switch_to will corrupt the FP context
   which has been initialized until flush_thread().

  To solve the 1st case, the initialization of the FP register will be
completed in start_thread(). It makes sure all FP registers are initialized
before starting the user program. For the 2nd case, the state of
reg->sstatus.FS in start_thread will be set to SR_FS_OFF to prevent this
process from corrupting FP context in doing context save. The FP state is
set to SR_FS_INITIAL in start_trhead().

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Fixes: 7db91e57a0 ("RISC-V: Task implementation")
Cc: stable@vger.kernel.org
[paul.walmsley@sifive.com: fixed brace alignment issue reported by
 checkpatch]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
This commit is contained in:
Vincent Chen 2019-08-14 16:23:52 +08:00 committed by Paul Walmsley
parent d568cb3f93
commit 8ac71d7e46
2 changed files with 15 additions and 2 deletions
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6
arch/riscv/include/asm/switch_to.h
View File

@ -19,6 +19,12 @@ static inline void __fstate_clean(struct pt_regs *regs)
regs->sstatus |= (regs->sstatus & ~(SR_FS)) | SR_FS_CLEAN;
}
static inline void fstate_off(struct task_struct *task,
struct pt_regs *regs)
{
regs->sstatus = (regs->sstatus & ~SR_FS) | SR_FS_OFF;
}
static inline void fstate_save(struct task_struct *task,
struct pt_regs *regs)
{

11
arch/riscv/kernel/process.c
View File

@ -64,8 +64,14 @@ void start_thread(struct pt_regs *regs, unsigned long pc,
unsigned long sp)
{
regs->sstatus = SR_SPIE;
if (has_fpu)
if (has_fpu) {
regs->sstatus |= SR_FS_INITIAL;
/*
* Restore the initial value to the FP register
* before starting the user program.
*/
fstate_restore(current, regs);
}
regs->sepc = pc;
regs->sp = sp;
set_fs(USER_DS);
@ -75,10 +81,11 @@ void flush_thread(void)
{
#ifdef CONFIG_FPU
/*
* Reset FPU context
* Reset FPU state and context
* frm: round to nearest, ties to even (IEEE default)
* fflags: accrued exceptions cleared
*/
fstate_off(current, task_pt_regs(current));
memset(&current->thread.fstate, 0, sizeof(current->thread.fstate));
#endif
}