RE: [RESEND PATCH 5/6] KVM: x86/VMX: add kvm_vmx_reinject_nmi_irq() for NMI/IRQ reinjection

Li, Xin3
2022-11-17T03:37:53Z

> > > > > But what about NMIs, afaict this is all horribly broken for NMIs.
> > > > >
> > > > > So the whole VMX thing latches the NMI (which stops NMI
> > > > > recursion),
> > > right?
> > > > >
> > > > > But then you drop out of noinstr code, which means any random
> > > > > exception can happen (kprobes #BP, hw_breakpoint #DB, or even
> > > > > #PF due to random nonsense like *SAN). This exception will do
> > > > > IRET and clear the NMI latch, all before you get to run any of the NMI
> code.
> > > >
> > > > What you said here implies that we have this problem in the existing
> code.
> > > > Because a fake iret stack is created to call the NMI handler in
> > > > the IDT NMI descriptor, which lastly executes the IRET instruction.
> > >
> > > I can't follow; of course the IDT handler terminates with IRET, it has to no?
> >
> > With FRED, ERETS/ERETU replace IRET, and use bit 28 of the popped CS
> > field to control whether to unblock NMI. If bit 28 of the field (above
> > the selector) is 1, ERETS/ERETU unblocks NMIs.
>
> Yes, I know that. It is one of the many improvements FRED brings.
> Ideally the IBT WAIT-FOR-ENDBR state also gets squirreled away in the
> hardware exception frame, but that's still up in the air I believe :/
>
> Anyway.. given there is interrupt priority and NMI is pretty much on top of
> everything else the reinject crap *should* run NMI first. That way NMI runs
> with the latch disabled and whatever other pending interrupts will run later.
>
> But that all is still broken because afaict the current code also leaves noinstr --
> and once you leave noinstr (or use a static_key, static_call or anything else that
> *can* change at runtime) you can't guarantee nothing.

For NMI, HPA asked me to use "int $2", as it switches to the NMI IST stack to
execute the NMI handler, essentially like how HW deals with a NMI in host. and
I tested it with NMI watchdog, it looks working fine.

For IRQs, we still use the dispatch table, but with a new func added in
DEFINE_IDTENTRY_SYSVEC with the noinstr entry/exit code removed:

#define DEFINE_IDTENTRY_SYSVEC(func) \
static void __##func(struct pt_regs *regs); \
\
__visible noinstr void func(struct pt_regs *regs) \
{ \
irqentry_state_t state = irqentry_enter(regs); \
\
instrumentation_begin(); \
kvm_set_cpu_l1tf_flush_l1d(); \
run_sysvec_on_irqstack_cond(__##func, regs); \
instrumentation_end(); \
irqentry_exit(regs, state); \
} \
\
+void dispatch_table_##func(struct pt_regs *regs) \
+{ \
+ kvm_set_cpu_l1tf_flush_l1d(); \
+ run_sysvec_on_irqstack_cond(__##func, regs); \
+}
+ \
\
static noinline void __##func(struct pt_regs *regs)

How do you think?