Re: [PATCH v4] i386: Add ratelimit for bus locks acquired in guest

[Chenyi Qiang]

On 5/28/2021 5:19 AM, Eduardo Habkost wrote:
> On Fri, May 21, 2021 at 12:38:20PM +0800, Chenyi Qiang wrote:
> [...]
> > @@ -4222,6 +4247,15 @@ void kvm_arch_pre_run(CPUState *cpu, struct kvm_run *run)
> >       }
> >   }
> >   
> > +static void kvm_rate_limit_on_bus_lock(void)
> > +{
> > +    uint64_t delay_ns = ratelimit_calculate_delay(&bus_lock_ratelimit_ctrl, 1);
> > +
> > +    if (delay_ns) {
> > +        g_usleep(delay_ns / SCALE_US);
> > +    }
> > +}
> > +
> >   MemTxAttrs kvm_arch_post_run(CPUState *cpu, struct kvm_run *run)
> >   {
> >       X86CPU *x86_cpu = X86_CPU(cpu);
> > @@ -4237,6 +4271,9 @@ MemTxAttrs kvm_arch_post_run(CPUState *cpu, struct 
> > kvm_run *run)
> >       } else {
> >           env->eflags &= ~IF_MASK;
> >       }
> > +    if (run->flags & KVM_RUN_X86_BUS_LOCK) {
>
> Does the KVM API guarantee that KVM_RUN_X86_BUS_LOCK will never
> be set if KVM_BUS_LOCK_DETECTION_EXIT isn't enabled?  (Otherwise
> we risk crashing in ratelimit_calculate_delay() above if rate
> limiting is disabled).

Yes. KVM_RUN_X86_BUS_LOCK flag is set when bus lock VM exit happens. Bus 
lock VM exit is disabled by default and can only be enabled through the 
KVM_BUS_LOCK_DETECTION_EXIT capability.

> If that's guaranteed, the patch looks good to me now.
>
> > +        kvm_rate_limit_on_bus_lock();
> > +    }
> >   
> >       /* We need to protect the apic state against concurrent accesses from
> >        * different threads in case the userspace irqchip is used. */
> > @@ -4595,6 +4632,10 @@ int kvm_arch_handle_exit(CPUState *cs, struct kvm_run 
> > *run)
> >           ioapic_eoi_broadcast(run->eoi.vector);
> >           ret = 0;
> >           break;
> > +    case KVM_EXIT_X86_BUS_LOCK:
> > +        /* already handled in kvm_arch_post_run */
> > +        ret = 0;
> > +        break;
> >       default:
> >           fprintf(stderr, "KVM: unknown exit reason %d\n", run->exit_reason);
> >           ret = -1;
> > --
> > 2.17.1