linux-kernel - Re: [PATCH 4/5] Powerpc/hw-breakpoint: Optimize disable path

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Message-ID: <ab7e5fac2a1ea78181900f5df7411b1f51b65eb9.camel@neuling.org>
Date:   Tue, 18 Jun 2019 16:15:11 +1000
From:   Michael Neuling <mikey@...ling.org>
To:     Ravi Bangoria <ravi.bangoria@...ux.ibm.com>, mpe@...erman.id.au
Cc:     benh@...nel.crashing.org, paulus@...ba.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        npiggin@...il.com, christophe.leroy@....fr,
        naveen.n.rao@...ux.vnet.ibm.com
Subject: Re: [PATCH 4/5] Powerpc/hw-breakpoint: Optimize disable path

On Tue, 2019-06-18 at 09:57 +0530, Ravi Bangoria wrote:
> Directly setting dawr and dawrx with 0 should be enough to
> disable watchpoint. No need to reset individual bits in
> variable and then set in hw.

This seems like a pointless optimisation to me. 

I'm all for adding more code/complexity if it buys us some performance, but I
can't imagine this is a fast path (nor have you stated any performance
benefits). 

Mikey

> 
> Signed-off-by: Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
> ---
>  arch/powerpc/include/asm/hw_breakpoint.h |  3 ++-
>  arch/powerpc/kernel/process.c            | 12 ++++++++++++
>  2 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/include/asm/hw_breakpoint.h
> b/arch/powerpc/include/asm/hw_breakpoint.h
> index 78202d5fb13a..8acbbdd4a2d5 100644
> --- a/arch/powerpc/include/asm/hw_breakpoint.h
> +++ b/arch/powerpc/include/asm/hw_breakpoint.h
> @@ -19,6 +19,7 @@ struct arch_hw_breakpoint {
>  /* Note: Don't change the the first 6 bits below as they are in the same
> order
>   * as the dabr and dabrx.
>   */
> +#define HW_BRK_TYPE_DISABLE		0x00
>  #define HW_BRK_TYPE_READ		0x01
>  #define HW_BRK_TYPE_WRITE		0x02
>  #define HW_BRK_TYPE_TRANSLATE		0x04
> @@ -68,7 +69,7 @@ static inline void hw_breakpoint_disable(void)
>  	struct arch_hw_breakpoint brk;
>  
>  	brk.address = 0;
> -	brk.type = 0;
> +	brk.type = HW_BRK_TYPE_DISABLE;
>  	brk.len = 0;
>  	if (ppc_breakpoint_available())
>  		__set_breakpoint(&brk);
> diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> index f002d2ffff86..265fac9fb3a4 100644
> --- a/arch/powerpc/kernel/process.c
> +++ b/arch/powerpc/kernel/process.c
> @@ -793,10 +793,22 @@ static inline int set_dabr(struct arch_hw_breakpoint
> *brk)
>  	return __set_dabr(dabr, dabrx);
>  }
>  
> +static int disable_dawr(void)
> +{
> +	if (ppc_md.set_dawr)
> +		return ppc_md.set_dawr(0, 0);
> +
> +	mtspr(SPRN_DAWRX, 0);
> +	return 0;
> +}
> +
>  int set_dawr(struct arch_hw_breakpoint *brk)
>  {
>  	unsigned long dawr, dawrx, mrd;
>  
> +	if (brk->type == HW_BRK_TYPE_DISABLE)
> +		return disable_dawr();
> +
>  	dawr = brk->address;
>  
>  	dawrx  = (brk->type & HW_BRK_TYPE_RDWR) << (63 - 58);