linux-kernel - Re: [PATCH] arm64: fix bug for reloading FPSIMD state after execve on cpu 0.

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Message-ID: <20150826111205.GD30466@arm.com>
Date:	Wed, 26 Aug 2015 12:12:05 +0100
From:	Will Deacon <will.deacon@....com>
To:	Chunyan Zhang <chunyan.zhang@...eadtrum.com>
Cc:	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"jianhua.ljh@...il.com" <jianhua.ljh@...il.com>,
	"orson.zhai@...eadtrum.com" <orson.zhai@...eadtrum.com>,
	"xiongshan.an@...eadtrum.com" <xiongshan.an@...eadtrum.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	ard.biesheuvel@...aro.org
Subject: Re: [PATCH] arm64: fix bug for reloading FPSIMD state after execve
 on cpu 0.

Hello,

[adding Ard]

On Wed, Aug 26, 2015 at 03:40:41AM +0100, Chunyan Zhang wrote:
> From: Janet Liu <janet.liu@...eadtrum.com>
> 
> If process A is running on CPU 0 and do execve syscall and after sched_exec,
> dest_cpu is 0, fpsimd_state.cpu is 0. If at the time Process A get scheduled
> out and after some kernel threads running on CPU 0, process A is back in CPU 0,
> A's fpsimd_state.cpu is current cpu id "0", and per_cpu(fpsimd_last_state)
> points A's fpsimd_state, TIF_FOREIGN_FPSTATE will be clear, kernel will not
> reload the context during it return to userspace. so set the cpu's
> fpsimd_last_state to NULL to avoid this.

AFAICT, this is only a problem if one of the kernel threads uses the fpsimd
registers, right? However, kernel_neon_begin_partial clobbers
fpsimd_last_state, so I'm struggling to see the problem.

Are you seeing an issue in practice?

Will

> Signed-off-by: Janet Liu <janet.liu@...eadtrum.com>
> Signed-off-by: Chunyan Zhang <chunyan.zhang@...eadtrum.com>
> ---
>  arch/arm64/kernel/fpsimd.c |    1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index 44d6f75..ec58d94 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -159,6 +159,7 @@ void fpsimd_flush_thread(void)
>  {
>  	memset(&current->thread.fpsimd_state, 0, sizeof(struct fpsimd_state));
>  	set_thread_flag(TIF_FOREIGN_FPSTATE);
> +	this_cpu_write(fpsimd_last_state, NULL);
>  }
>  
>  /*
> -- 
> 1.7.9.5
> 
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