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

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Message-ID: <52458604.7030902@gmail.com>
Date:	Fri, 27 Sep 2013 21:20:04 +0800
From:	Jiang Liu <liuj97@...il.com>
To:	Catalin Marinas <catalin.marinas@....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 09/27/2013 07:35 PM, Catalin Marinas wrote:
> 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.
OK, will drop it.

> 
>>  }
>>  
>> @@ -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.
OK, will keep FPCR and clear FPSR.

> 
>>  	/* 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.
> 

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