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

Frederic Weisbecker
2020-02-25T16:21:25Z

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.
>
> 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 you're interrupted here and __ctx->cnt == 0, the new NMI is in its right
to overwrite __ctx->hcr. It will find HCR_TGE set in the sysreg and write it back to
___ctx->hcr. So the following restore will fail.

\
> if (!(___ctx->hcr & HCR_TGE)) \
> write_sysreg(___ctx->hcr, hcr_el2); \
> } while (0)