[Qemu-devel] Re: [PATCH 08/13] kvm: x86: Inject pending MCE events on state writeback

[Marcelo Tosatti]

On Thu, Feb 17, 2011 at 06:06:19PM +0100, Jan Kiszka wrote:
> On 2011-02-17 17:35, Marcelo Tosatti wrote:
> > On Tue, Feb 15, 2011 at 09:23:32AM +0100, Jan Kiszka wrote:
> >> The current way of injecting MCE events without updating of and
> >> synchronizing with the CPUState is broken and causes spurious
> >> corruptions of the MCE-related parts of the CPUState.
> > 
> > Can you explain how? The current pronlem with MCE is that it bypasses 
> > writeback code, but corruption has nothing to do with that.
> 
> It's precisely the same scenario as with the old debug exception
> re-injection: If we update the pending exception state via
> KVM_SET_VCPU_EVENTS, we must not inject it via any other path. Otherwise
> we end up with overwritten/lost events - which is extremely critical for
> this rarely taken code paths.
> 
> Jut like parts of KVM_SET_GUEST_DEBUG, KVM_X86_SET_MCE pre-dates
> KVM_SET_VCPU_EVENTS which obsoleted all other exception injection
> mechanisms.

OK.

> > 
> >> As a first step towards a fix, enhance the state writeback code with
> >> support for injecting events that are pending in the CPUState. A pending
> >> exception will then be signaled via cpu_interrupt(CPU_INTERRUPT_MCE).
> >> And, just like for TCG, we need to leave the halt state when
> >> CPU_INTERRUPT_MCE is pending (left broken for the to-be-removed old KVM
> >> code).
> >>
> >> This will also allow to unify TCG and KVM injection code.
> >>
> >> Signed-off-by: Jan Kiszka <address@hidden>
> >> CC: Huang Ying <address@hidden>
> >> CC: Hidetoshi Seto <address@hidden>
> >> CC: Jin Dongming <address@hidden>
> >> ---
> >>  target-i386/kvm.c |   75 
> >> +++++++++++++++++++++++++++++++++++++++++++++++++---
> >>  1 files changed, 70 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/target-i386/kvm.c b/target-i386/kvm.c
> >> index f909661..46f45db 100644
> >> --- a/target-i386/kvm.c
> >> +++ b/target-i386/kvm.c
> >> @@ -467,6 +467,44 @@ void kvm_inject_x86_mce(CPUState *cenv, int bank, 
> >> uint64_t status,
> >>  #endif /* !KVM_CAP_MCE*/
> >>  }
> >>  
> >> +static int kvm_inject_mce_oldstyle(CPUState *env)
> >> +{
> >> +#ifdef KVM_CAP_MCE
> >> +    if (kvm_has_vcpu_events()) {
> >> +        return 0;
> >> +    }
> >> +    if (env->interrupt_request & CPU_INTERRUPT_MCE) {
> >> +        unsigned int bank, bank_num = env->mcg_cap & 0xff;
> >> +        struct kvm_x86_mce mce;
> >> +
> >> +        /* We must not raise CPU_INTERRUPT_MCE if it's not supported. */
> >> +        assert(env->mcg_cap);
> >> +
> >> +        env->interrupt_request &= ~CPU_INTERRUPT_MCE;
> >> +
> >> +        /*
> >> +         * There must be at least one bank in use if CPU_INTERRUPT_MCE 
> >> was set.
> >> +         * Find it and use its values for the event injection.
> >> +         */
> >> +        for (bank = 0; bank < bank_num; bank++) {
> >> +            if (env->mce_banks[bank * 4 + 1] & MCI_STATUS_VAL) {
> >> +                break;
> >> +            }
> >> +        }
> >> +        assert(bank < bank_num);
> >> +
> >> +        mce.bank = bank;
> >> +        mce.status = env->mce_banks[bank * 4 + 1];
> >> +        mce.mcg_status = env->mcg_status;
> >> +        mce.addr = env->mce_banks[bank * 4 + 2];
> >> +        mce.misc = env->mce_banks[bank * 4 + 3];
> >> +
> >> +        return kvm_vcpu_ioctl(env, KVM_X86_SET_MCE, &mce);
> >> +    }
> >> +#endif /* KVM_CAP_MCE */
> >> +    return 0;
> >> +}
> >> +
> >>  static void cpu_update_state(void *opaque, int running, int reason)
> >>  {
> >>      CPUState *env = opaque;
> >> @@ -1375,10 +1413,25 @@ static int kvm_put_vcpu_events(CPUState *env, int 
> >> level)
> >>          return 0;
> >>      }
> >>  
> >> -    events.exception.injected = (env->exception_injected >= 0);
> >> -    events.exception.nr = env->exception_injected;
> >> -    events.exception.has_error_code = env->has_error_code;
> >> -    events.exception.error_code = env->error_code;
> >> +    if (env->interrupt_request & CPU_INTERRUPT_MCE) {
> >> +        /* We must not raise CPU_INTERRUPT_MCE if it's not supported. */
> >> +        assert(env->mcg_cap);
> >> +
> >> +        env->interrupt_request &= ~CPU_INTERRUPT_MCE;
> >> +        if (env->exception_injected == EXCP08_DBLE) {
> >> +            /* this means triple fault */
> >> +            qemu_system_reset_request();
> >> +            env->exit_request = 1;
> >> +        }
> >> +        events.exception.injected = 1;
> >> +        events.exception.nr = EXCP12_MCHK;
> >> +        events.exception.has_error_code = 0;
> >> +    } else {
> >> +        events.exception.injected = (env->exception_injected >= 0);
> >> +        events.exception.nr = env->exception_injected;
> >> +        events.exception.has_error_code = env->has_error_code;
> >> +        events.exception.error_code = env->error_code;
> >> +    }
> > 
> > IMO it is important to maintain a scope for kvm_put_vcpu_events /
> > kvm_get_vcpu_events: they synchronize state to/from the kernel. Not more
> > than that. Whatever you're trying to do here should be higher in the
> > vcpu loop code.
> 
> We pick up CPU_INTERRUPT_MCE and translate it into the right exception
> that put_vcpu_events is about to sync to the kernel. What should be done
> earlier of those steps? Calculating env->exception_injected?

Everything but writeback. Update env->exception_injected/nr in
process_irqchip_events, or in a separate kvm_arch_update_exceptions.

> >>          return ret;
> >> @@ -1678,10 +1736,17 @@ void kvm_arch_post_run(CPUState *env, struct 
> >> kvm_run *run)
> >>  int kvm_arch_process_irqchip_events(CPUState *env)
> >>  {
> >>      if (kvm_irqchip_in_kernel()) {
> >> +        if (env->interrupt_request & CPU_INTERRUPT_MCE) {
> >> +            kvm_cpu_synchronize_state(env);
> >> +            if (env->mp_state == KVM_MP_STATE_HALTED) {
> >> +                env->mp_state = KVM_MP_STATE_RUNNABLE;
> >> +            }
> >> +        }
> > 
> > Should not manipulate mp_state of a running vcpu (should only do that
> > for migration when vcpu is stopped), since its managed by the kernel,
> > for irqchip case.
> 
> Not for asynchronously injected MCEs. The target CPU would simply
> oversleep them. MCEs are not in the scope of the in-kernel irqchip.

Pending MCE exception could break out of in-kernel halt emulation.