On Thu, Aug 8, 2019 at 1:30 PM Vincent Chen <vincent.c...@sifive.com> wrote:
>
> 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().
>
> Tested on both QEMU and HiFive Unleashed using BBL + Linux.
>
> Signed-off-by: Vincent Chen <vincent.c...@sifive.com>
> ---
> arch/riscv/include/asm/switch_to.h | 6 ++++++
> arch/riscv/kernel/process.c | 13 +++++++++++--
> 2 files changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/include/asm/switch_to.h
> b/arch/riscv/include/asm/switch_to.h
> index 853b65e..d5fe573 100644
> --- a/arch/riscv/include/asm/switch_to.h
> +++ b/arch/riscv/include/asm/switch_to.h
> @@ -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;
The SR_FS_OFF is 0x0 so no need for ORing it.
> +}
> +
> static inline void fstate_save(struct task_struct *task,
> struct pt_regs *regs)
> {
> diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
> index f23794b..e3077ee 100644
> --- a/arch/riscv/kernel/process.c
> +++ b/arch/riscv/kernel/process.c
> @@ -64,8 +64,16 @@ 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;
> +#ifdef CONFIG_FPU
> + /*
> + * Restore the initial value to the FP register
> + * before starting the user program.
> + */
> + fstate_restore(current, regs);
> +#endif
> + }
> regs->sepc = pc;
> regs->sp = sp;
> set_fs(USER_DS);
> @@ -75,10 +83,11 @@ 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(¤t->thread.fstate, 0, sizeof(current->thread.fstate));
> #endif
> }
> --
> 2.7.4
>
Apart from above minor comment, looks good to me.
Reviewed-by: Anup Patel <a...@brainfault.org>
Regards,
Anup
