Re: [PATCH 12/17] arm64: debug-monitors: refactor MDSCR manipulation

From: Anshuman Khandual <anshuman.khandual@arm.com>
To: Mark Rutland <mark.rutland@arm.com>,
	linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com,
	will@kernel.org, james.morse@arm.com
Cc: keescook@chromium.org, maz@kernel.org, broonie@kernel.org,
	labbott@redhat.com, robin.murphy@arm.com,
	julien.thierry.kdev@gmail.com, alex.popov@linux.com
Subject: Re: [PATCH 12/17] arm64: debug-monitors: refactor MDSCR manipulation
Date: Fri, 10 Jan 2020 10:05:48 +0530	[thread overview]
Message-ID: <c616e5c0-301c-7c26-6954-9e287e6c9673@arm.com> (raw)
In-Reply-To: <20200108185634.1163-13-mark.rutland@arm.com>

On 01/09/2020 12:26 AM, Mark Rutland wrote:
> When we convert the ret_to_user/work_pending code to C, we're going to
> want to poke the MDSCR to enable/disable single-step. Let's factor out
> the existing code for this from debug-monitors.c.
> 
> At the same time, we can make use of {read,write}_sysreg() directly, and
> get rid of the mdscr_{read,write} wrappers.
> 
> The existing code masked DAIF when manipulating MDSCR_EL1, but this
> should not be necessary. Exceptions can be taken immediately before DAIF
> is masked, and given the lack of an ISB can also be taken after DAIF is
> unmasked as writes to DAIF are only self-synchronizing and not
> context-synchronizing in general. We may want to add an ISB to ensure
> that updates to MDSCR have taken effect, however.

Any reason this patch choose not add that ISB for now after writing
mdscr_el1 register via sysreg_clear_set().

> 
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> ---
>  arch/arm64/include/asm/debug-monitors.h | 10 ++++++++++
>  arch/arm64/kernel/debug-monitors.c      | 32 +++++++-------------------------
>  2 files changed, 17 insertions(+), 25 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/debug-monitors.h b/arch/arm64/include/asm/debug-monitors.h
> index 7619f473155f..342867e50c54 100644
> --- a/arch/arm64/include/asm/debug-monitors.h
> +++ b/arch/arm64/include/asm/debug-monitors.h
> @@ -107,6 +107,16 @@ enum dbg_active_el {
>  void enable_debug_monitors(enum dbg_active_el el);
>  void disable_debug_monitors(enum dbg_active_el el);
>  
> +static __always_inline void __enable_single_step_nosync(void)
> +{
> +	sysreg_clear_set(mdscr_el1, 0, DBG_MDSCR_SS);
> +}
> +
> +static __always_inline void __disable_single_step_nosync(void)
> +{
> +	sysreg_clear_set(mdscr_el1, DBG_MDSCR_SS, 0);
> +}
> +
>  void user_rewind_single_step(struct task_struct *task);
>  void user_fastforward_single_step(struct task_struct *task);
>  
> diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> index 48222a4760c2..fa2d4145bd07 100644
> --- a/arch/arm64/kernel/debug-monitors.c
> +++ b/arch/arm64/kernel/debug-monitors.c
> @@ -32,24 +32,6 @@ u8 debug_monitors_arch(void)
>  }
>  
>  /*
> - * MDSCR access routines.
> - */
> -static void mdscr_write(u32 mdscr)
> -{
> -	unsigned long flags;
> -	flags = local_daif_save();
> -	write_sysreg(mdscr, mdscr_el1);
> -	local_daif_restore(flags);
> -}
> -NOKPROBE_SYMBOL(mdscr_write);
> -
> -static u32 mdscr_read(void)
> -{
> -	return read_sysreg(mdscr_el1);
> -}
> -NOKPROBE_SYMBOL(mdscr_read);
> -
> -/*
>   * Allow root to disable self-hosted debug from userspace.
>   * This is useful if you want to connect an external JTAG debugger.
>   */
> @@ -91,9 +73,9 @@ void enable_debug_monitors(enum dbg_active_el el)
>  		enable |= DBG_MDSCR_KDE;
>  
>  	if (enable && debug_enabled) {
> -		mdscr = mdscr_read();
> +		mdscr = read_sysreg(mdscr_el1);
>  		mdscr |= enable;
> -		mdscr_write(mdscr);
> +		write_sysreg(mdscr, mdscr_el1);
>  	}
>  }
>  NOKPROBE_SYMBOL(enable_debug_monitors);
> @@ -112,9 +94,9 @@ void disable_debug_monitors(enum dbg_active_el el)
>  		disable &= ~DBG_MDSCR_KDE;
>  
>  	if (disable) {
> -		mdscr = mdscr_read();
> +		mdscr = read_sysreg(mdscr_el1);
>  		mdscr &= disable;
> -		mdscr_write(mdscr);
> +		write_sysreg(mdscr, mdscr_el1);
>  	}
>  }
>  NOKPROBE_SYMBOL(disable_debug_monitors);
> @@ -409,7 +391,7 @@ void kernel_enable_single_step(struct pt_regs *regs)
>  {
>  	WARN_ON(!irqs_disabled());
>  	set_regs_spsr_ss(regs);
> -	mdscr_write(mdscr_read() | DBG_MDSCR_SS);
> +	__enable_single_step_nosync();
>  	enable_debug_monitors(DBG_ACTIVE_EL1);
>  }
>  NOKPROBE_SYMBOL(kernel_enable_single_step);
> @@ -417,7 +399,7 @@ NOKPROBE_SYMBOL(kernel_enable_single_step);
>  void kernel_disable_single_step(void)
>  {
>  	WARN_ON(!irqs_disabled());
> -	mdscr_write(mdscr_read() & ~DBG_MDSCR_SS);
> +	__disable_single_step_nosync();
>  	disable_debug_monitors(DBG_ACTIVE_EL1);
>  }
>  NOKPROBE_SYMBOL(kernel_disable_single_step);
> @@ -425,7 +407,7 @@ NOKPROBE_SYMBOL(kernel_disable_single_step);
>  int kernel_active_single_step(void)
>  {
>  	WARN_ON(!irqs_disabled());
> -	return mdscr_read() & DBG_MDSCR_SS;
> +	return read_sysreg(mdscr_el1) & DBG_MDSCR_SS;
>  }
>  NOKPROBE_SYMBOL(kernel_active_single_step);
>  
> 

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