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

[Frederic Weisbecker]

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...