Re: [PATCH v4 02/27] hardirq/nmi: Allow nested nmi_enter()

Peter Zijlstra
2020-02-25T15:42:00Z

On Tue, Feb 25, 2020 at 04:09:06AM +0100, Frederic Weisbecker wrote:
> On Mon, Feb 24, 2020 at 05:13:18PM +0100, Peter Zijlstra wrote:

> > +#define arch_nmi_enter() \
> > +do { \
> > + struct nmi_ctx *___ctx; \
> > + unsigned int ___cnt; \
> > + \
> > + if (!is_kernel_in_hyp_mode() || in_nmi()) \
> > + break; \
> > + \
> > + ___ctx = this_cpu_ptr(&nmi_contexts); \
> > + ___cnt = ___ctx->cnt; \
> > + if (!(___cnt & 1) && __cnt) { \
> > + ___ctx->cnt += 2; \
> > + break; \
> > + } \
> > + \
> > + ___ctx->cnt |= 1; \
> > + barrier(); \
> > + nmi_ctx->hcr = read_sysreg(hcr_el2); \
> > + if (!(nmi_ctx->hcr & HCR_TGE)) { \
> > + write_sysreg(nmi_ctx->hcr | HCR_TGE, hcr_el2); \
> > + isb(); \
> > + } \
> > + barrier(); \
>
> Suppose the first NMI is interrupted here. nmi_ctx->hcr has HCR_TGE unset.
> The new NMI is going to overwrite nmi_ctx->hcr with HCR_TGE set. Then the
> first NMI will not restore the correct value upon arch_nmi_exit().
>
> So perhaps the below, but I bet I overlooked something obvious.

Well, none of this is obvious :/

The basic idea was that the LSB signifies 'pending/in-progress' and when
that is set, nobody else touches no nothing. Enter will unconditionally
(re) write_sysreg(), exit will nothing.

Obviously I messed that up.

How's this?

#define arch_nmi_enter() \
do { \
struct nmi_ctx *___ctx; \
unsigned int ___cnt; \
\
if (!is_kernel_in_hyp_mode() || in_nmi()) \
break; \
\
___ctx = this_cpu_ptr(&nmi_contexts); \
___cnt = ___ctx->cnt; \
if (!(___cnt & 1)) { /* !IN-PROGRESS */ \
if (___cnt) { \
___ctx->cnt += 2; \
break; \
} \
\
___ctx->hcr = read_sysreg(hcr_el2); \
barrier(); \
___ctx->cnt |= 1; /* IN-PROGRESS */ \
barrier(); \
} \
\
if (!(___ctx->hcr & HCR_TGE)) { \
write_sysreg(___ctx->hcr | HCR_TGE, hcr_el2); \
isb(); \
} \
barrier(); \
if (!(___cnt & 1)) \
___ctx->cnt++; /* COMPLETE */ \
} while (0)

#define arch_nmi_exit() \
do { \
struct nmi_ctx *___ctx; \
\
if (!is_kernel_in_hyp_mode() || in_nmi()) \
break; \
\
___ctx = this_cpu_ptr(&nmi_contexts); \
if ((___ctx->cnt & 1) || (___ctx->cnt -= 2)) \
break; \
\
if (!(___ctx->hcr & HCR_TGE)) \
write_sysreg(___ctx->hcr, hcr_el2); \
} while (0)