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

Frederic Weisbecker
2020-02-25T22:10:36Z

On Tue, Feb 25, 2020 at 04:41:11PM +0100, Peter Zijlstra wrote:
> 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.

So here is my previous proposal, based on a simple counter, this time
with comments and a few fixes:

#define arch_nmi_enter() \
do { \
struct nmi_ctx *___ctx; \
u64 ___hcr; \
\
if (!is_kernel_in_hyp_mode()) \
break; \
\
___ctx = this_cpu_ptr(&nmi_contexts); \
if (___ctx->cnt) { \
___ctx->cnt++; \
break; \
} \
\
___hcr = read_sysreg(hcr_el2); \
if (!(___hcr & HCR_TGE)) { \
write_sysreg(___hcr | HCR_TGE, hcr_el2); \
isb(); \
} \
/* \
* Make sure the sysreg write is performed before ___ctx->cnt \
* is set to 1. NMIs that see cnt == 1 will rely on us. \
*/ \
barrier(); \
___ctx->cnt = 1; \
/* \
* Make sure ___ctx->cnt is set before we save ___hcr. We \
* don't want ___ctx->hcr to be overwritten. \
*/ \
barrier(); \
___ctx->hcr = ___hcr; \
} while (0)

#define arch_nmi_exit() \
do { \
struct nmi_ctx *___ctx; \
u64 ___hcr; \
\
if (!is_kernel_in_hyp_mode()) \
break; \
\
___ctx = this_cpu_ptr(&nmi_contexts); \
___hcr = ___ctx->hcr; \
/* \
* Make sure we read ___ctx->hcr before we release \
* ___ctx->cnt as it makes ___ctx->hcr updatable again. \
*/ \
barrier(); \
___ctx->cnt--; \
/* \
* Make sure ___ctx->cnt release is visible before we \
* restore the sysreg. Otherwise a new NMI occuring \
* 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); \
} while (0)