linux-kernel - Re: [RFT PATCH v1 2/7] arm64: restore FPSIMD to default state for kernel and signal contexts

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Date:	Fri, 27 Sep 2013 12:35:07 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Jiang Liu <liuj97@...il.com>
Cc:	Will Deacon <Will.Deacon@....com>,
	Jiang Liu <jiang.liu@...wei.com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFT PATCH v1 2/7] arm64: restore FPSIMD to default state for
 kernel and signal contexts

On Fri, Sep 27, 2013 at 09:04:42AM +0100, Jiang Liu wrote:
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index bb785d2..12a25e5 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -80,9 +80,14 @@ void fpsimd_thread_switch(struct task_struct *next)
>  
>  void fpsimd_flush_thread(void)
>  {
> +	struct fpsimd_state *state = &current->thread.fpsimd_state;
> +
>  	preempt_disable();
> -	memset(&current->thread.fpsimd_state, 0, sizeof(struct fpsimd_state));
> -	fpsimd_load_state(&current->thread.fpsimd_state);
> +	memset(state, 0, sizeof(struct fpsimd_state));
> +#if (AARCH64_FPCR_DEFAULT_VAL != 0)
> +	state->fpcr = AARCH64_FPCR_DEFAULT_VAL;
> +#endif
> +	fpsimd_load_state(state);
>  	preempt_enable();

We can leave this to 0 as the default for user. Glibc already programs
this register.

>  }
>  
> @@ -99,6 +104,8 @@ void kernel_neon_begin(void)
>  
>  	if (current->mm)
>  		fpsimd_save_state(&current->thread.fpsimd_state);
> +
> +	fpsimd_init_hw_state();

This function should indeed do some initialisation rather than relying
on whatever the user set for FPCR.

> diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
> index 890a591..6d80612 100644
> --- a/arch/arm64/kernel/signal.c
> +++ b/arch/arm64/kernel/signal.c
> @@ -52,6 +52,7 @@ static int preserve_fpsimd_context(struct fpsimd_context __user *ctx)
>  
>  	/* dump the hardware registers to the fpsimd_state structure */
>  	fpsimd_save_state(fpsimd);
> +	fpsimd_init_hw_state();

Here I think we need to leave the default user setting for fpcr as
decided by glibc (or libgcc) when an application starts.

>  	/* copy the FP and status/control registers */
>  	err = __copy_to_user(ctx->vregs, fpsimd->vregs, sizeof(fpsimd->vregs));
> diff --git a/arch/arm64/kernel/signal32.c b/arch/arm64/kernel/signal32.c
> index e393174..cb2cb41 100644
> --- a/arch/arm64/kernel/signal32.c
> +++ b/arch/arm64/kernel/signal32.c
> @@ -248,6 +248,7 @@ static int compat_preserve_vfp_context(struct compat_vfp_sigframe __user *frame)
>  	 * in AArch32.
>  	 */
>  	fpsimd_save_state(fpsimd);
> +	fpsimd_init_hw_state();

Same here.

-- 
Catalin
--
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