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

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Jan 10, 2020 at 10:05:48AM +0530, Anshuman Khandual wrote:
> 
> 
> 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().

I didn't want to make that functional change without justification. For
example, the ISB wouldn't be needed for changes that only affect
userspace.

Thanks,
Mark.

> 
> > 
> > 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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