Re: [PATCH 04/17] arm64: entry: move preempt logic to C

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

On Thu, Jan 09, 2020 at 12:13:13PM +0530, Anshuman Khandual wrote:
> On 01/09/2020 12:26 AM, Mark Rutland wrote:
> > Some of our preeemption logic is in C, while a portion of it is in
> > assembly. Let's pull the remainder  of it into C so that it lives in one
> > place.
> > 
> > At the same time, let's improve the comments regarding NMI preemption to
> > make it clearer why we cannot preempt from NMIs.
> > 
> > Subsequent patches will covert the caller of el1_preempt() to C.
> > 
> > Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > Cc: James Morse <james.morse@arm.com>
> > Cc: Julien Thierry <julien.thierry.kdev@gmail.com>
> > Cc: Will Deacon <will@kernel.org>
> > ---
> >  arch/arm64/kernel/entry-common.c | 18 +++++++++++++++++-
> >  arch/arm64/kernel/entry.S        | 13 +------------
> >  2 files changed, 18 insertions(+), 13 deletions(-)
> > 
> > diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> > index 4fa058453468..b93ca2148a20 100644
> > --- a/arch/arm64/kernel/entry-common.c
> > +++ b/arch/arm64/kernel/entry-common.c
> > @@ -8,6 +8,7 @@
> >  #include <linux/context_tracking.h>
> >  #include <linux/linkage.h>
> >  #include <linux/lockdep.h>
> > +#include <linux/preempt.h>
> >  #include <linux/ptrace.h>
> >  #include <linux/sched/debug.h>
> >  #include <linux/thread_info.h>
> > @@ -334,8 +335,23 @@ asmlinkage void notrace el0_sync_compat_handler(struct pt_regs *regs)
> >  NOKPROBE_SYMBOL(el0_sync_compat_handler);
> >  #endif /* CONFIG_COMPAT */
> >  
> > -asmlinkage void __sched arm64_preempt_schedule_irq(void)
> > +asmlinkage void __sched el1_preempt(void)
> >  {
> > +	if (!IS_ENABLED(CONFIG_PREEMPT) || preempt_count())
> > +		return;
> 
> IS_ENABLED(CONFIG_PREEMPT) is not really required here as the single
> call site for el1_preempt() is still wrapped around CONFIG_PREEMPT.
> So we should retain any one of them.

Using both was deliberate.

I wanted to avoid ifdeffery here, but also wanted to avoid the
unnecessary bits being build for !CONFIG_PREEMPT buillds, in both this
function and the caller.

In a subsequent patch this'll be made static, and called
unconditionally, where we'll definitely need to IS_ENABLED(). I'll
update the commit message to make this part explicit.

> 
> > +
> > +	/*
> > +	 * To avoid nesting NMIs and overflowing the stack, we must leave NMIs
> > +	 * masked until the exception return. We want to context-switch with
> > +	 * IRQs masked but NMIs enabled, so cannot preempt an NMI.
> > +	 *
> > +	 * PSTATE.{D,A,F} are cleared for IRQ and NMI by el1_irq().
> > +	 * When gic_handle_irq() handles an NMI, it leaves PSTATE.I set.
> > +	 * If anything is set in DAIF, this is an NMI.
> > +	 */
> > +	if (system_uses_irq_prio_masking() && read_sysreg(daif) != 0)
> 
> In case above CONFIG_PREEMPT gets dropped, preempt_count() can be
> moved here as well.

As above, I'm going to leave this as-is.

I also think it's worth keeping this separate from the early return for
!preempt_count() due to the comment, which is specific to the GIC prio
masking logic.

Thanks,
Mark.

> 
> > +		return;
> > +
> >  	lockdep_assert_irqs_disabled();
> >  
> >  	/*
> > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> > index 7c6a0a41676f..53ce1877a4aa 100644
> > --- a/arch/arm64/kernel/entry.S
> > +++ b/arch/arm64/kernel/entry.S
> > @@ -606,18 +606,7 @@ el1_irq:
> >  	irq_handler
> >  
> >  #ifdef CONFIG_PREEMPT
> > -	ldr	x24, [tsk, #TSK_TI_PREEMPT]	// get preempt count
> > -alternative_if ARM64_HAS_IRQ_PRIO_MASKING
> > -	/*
> > -	 * DA_F were cleared at start of handling. If anything is set in DAIF,
> > -	 * we come back from an NMI, so skip preemption
> > -	 */
> > -	mrs	x0, daif
> > -	orr	x24, x24, x0
> > -alternative_else_nop_endif
> > -	cbnz	x24, 1f				// preempt count != 0 || NMI return path
> > -	bl	arm64_preempt_schedule_irq	// irq en/disable is done inside
> > -1:
> > +	bl	el1_preempt
> >  #endif
> >  
> >  #ifdef CONFIG_ARM64_PSEUDO_NMI
> > 

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