linux-kernel - Re: [RFT PATCH v2 2/4] arm64: restore FPSIMD to default state for kernel and signal contexts

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Date:	Mon, 14 Oct 2013 23:30:00 +0800
From:	Jiang Liu <liuj97@...il.com>
To:	Will Deacon <will.deacon@....com>
CC:	Catalin Marinas <Catalin.Marinas@....com>,
	Jiang Liu <jiang.liu@...wei.com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"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 v2 2/4] arm64: restore FPSIMD to default state for
 kernel and signal contexts

On 10/14/2013 11:16 PM, Will Deacon wrote:
> On Sun, Oct 13, 2013 at 03:20:18PM +0100, Jiang Liu wrote:
>> From: Jiang Liu <jiang.liu@...wei.com>
>>
>> Restore FPSIMD control and status registers to default values
>> when creating new FPSIMD contexts for kernel context and reset
>> FPSIMD status register when creating FPSIMD context for signal
>> handling, otherwise the stale value in FPSIMD control and status
>> registers may affect the new kernal or signal handling contexts.
>>
>> Signed-off-by: Jiang Liu <jiang.liu@...wei.com>
>> Cc: Jiang Liu <liuj97@...il.com>
>> ---
>>  arch/arm64/include/asm/fpsimd.h | 16 ++++++++++++++++
>>  arch/arm64/kernel/fpsimd.c      | 11 +++++++++--
>>  arch/arm64/kernel/signal.c      |  1 +
>>  arch/arm64/kernel/signal32.c    |  1 +
>>  4 files changed, 27 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h
>> index c43b4ac..b2dc30f 100644
>> --- a/arch/arm64/include/asm/fpsimd.h
>> +++ b/arch/arm64/include/asm/fpsimd.h
>> @@ -50,8 +50,24 @@ struct fpsimd_state {
>>  #define VFP_STATE_SIZE		((32 * 8) + 4)
>>  #endif
>>  
>> +#define	AARCH64_FPCR_DEFAULT_VAL	0
>> +
>>  struct task_struct;
>>  
>> +static inline void fpsimd_init_hw_state(void)
>> +{
>> +	int val = AARCH64_FPCR_DEFAULT_VAL;
>> +
>> +	asm ("msr fpcr, %x0\n"
>> +	     "msr fpsr, xzr\n"
>> +             : : "r"(val));
>> +}
>> +
>> +static inline void fpsimd_clear_fpsr(void)
>> +{
>> +	asm ("msr fpsr, xzr\n");
>> +}
> 
> You have pretty weak asm constraints here...
Hi Will,
	We will add an explicit "volatile" here. But according to GCC docs, it
should have the same effect:
An asm instruction without any output operands is treated identically to
a volatile asm instruction.
Thanks!
Gerry

> 
>>  extern void fpsimd_save_state(struct fpsimd_state *state);
>>  extern void fpsimd_load_state(struct fpsimd_state *state);
>>  
>> 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();
>>  }
>>  
>> @@ -99,6 +104,8 @@ void kernel_neon_begin(void)
>>  
>>  	if (current->mm)
>>  		fpsimd_save_state(&current->thread.fpsimd_state);
>> +
>> +	fpsimd_init_hw_state();
>>  }
>>  EXPORT_SYMBOL(kernel_neon_begin);
>>  
>> diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
>> index 890a591..4ee231e 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_clear_fpsr();
> 
> ... so I reckon GCC could reorder these two calls, resulting in corruption
> of the saved state register.
> 
> Will
> 

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