Re: [patch V4 part 1 27/36] arm64: Prepare arch_nmi_enter() for recursion

[Mathieu Desnoyers]

----- On May 15, 2020, at 11:45 AM, Will Deacon will@xxxxxxxxxx wrote:

> On Fri, May 15, 2020 at 04:04:39PM +0200, Frederic Weisbecker wrote:
>> On Wed, May 13, 2020 at 07:28:34PM -0400, Mathieu Desnoyers wrote:
>> > ----- On May 5, 2020, at 9:16 AM, Thomas Gleixner tglx@xxxxxxxxxxxxx wrote:
>> > 
>> > > +#define arch_nmi_enter()						\
>> > [...]							\
>> > > +	___hcr = read_sysreg(hcr_el2);					\
>> > > +	if (!(___hcr & HCR_TGE)) {					\
>> > > +		write_sysreg(___hcr | HCR_TGE, hcr_el2);		\
>> > > +		isb();							\
>> > 
>> > Why is there an isb() above ^ ....
>> > 
>> > > +	}								\
>> > > +	/*								\
>> > [...]
>> > > -#define arch_nmi_exit()								\
>> > [...]
>> > > +	/*								\
>> > > +	 * Make sure ___ctx->cnt release is visible before we		\
>> > > +	 * restore the sysreg. Otherwise a new NMI occurring		\
>> > > +	 * right after write_sysreg() can be fooled and think		\
>> > > +	 * we secured things for it.					\
>> > > +	 */								\
>> > > +	barrier();							\
>> > > +	if (!___ctx->cnt && !(___hcr & HCR_TGE))			\
>> > > +		write_sysreg(___hcr, hcr_el2);				\
>> > 
>> > And not here ?
>> 
>> I have to defer to Will on this detail...
> 
> I think it's because we have to make sure that the register update has
> taken effect before we can safely run the NMI handler (and so an ISB is
> needed), but on the return path the exception return back to the interrupted
> context has an implicit ISB so there's no need for an extra one here.
> 
> Make sense?

Sure, as long as instructions executed between write_sysreg() and return
from exception do not care, which I think should be the case.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com