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From:   Anup Patel <anup@brainfault.org>
Date:   Thu, 8 Aug 2019 15:45:00 +0530
Message-ID: <CAAhSdy0BNN4G270WJ+OqrFAv3-z9o2iE+QDHHo-FY0fqh5wGqg@mail.gmail.com>
Subject: Re: [PATCH 1/2] riscv: Correct the initialized flow of FP register
To:     Vincent Chen <vincent.chen@sifive.com>
Cc:     Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        "linux-kernel@vger.kernel.org List" <linux-kernel@vger.kernel.org>
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On Thu, Aug 8, 2019 at 1:30 PM Vincent Chen <vincent.chen@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.chen@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(&current->thread.fstate, 0, sizeof(current->thread.fstate));
>  #endif
>  }
> --
> 2.7.4
>

Apart from above minor comment, looks good to me.

Reviewed-by: Anup Patel <anup@brainfault.org>

Regards,
Anup