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SubjectRe: [RFC v2 2/2] KVM: VMX: Enable bus lock VM exit
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On 9/3/2020 6:44 AM, Sean Christopherson wrote:
> On Tue, Sep 01, 2020 at 10:43:12AM +0200, Vitaly Kuznetsov wrote:
>>> @@ -6809,6 +6824,19 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
>>> if (unlikely(vmx->exit_reason.failed_vmentry))
>>> return EXIT_FASTPATH_NONE;
>>>
>>> + /*
>>> + * check the exit_reason to see if there is a bus lock
>>> + * happened in guest.
>>> + */
>>> + if (kvm_bus_lock_exit_enabled(vmx->vcpu.kvm)) {
>>> + if (vmx->exit_reason.bus_lock_detected) {
>>> + vcpu->stat.bus_locks++;
>
> Why bother with stats? Every bus lock exits to userspace, having quick
> stats doesn't seem all that interesting.

OK. We will remove it.

>>> + vcpu->arch.bus_lock_detected = true;
>>> + } else {
>>> + vcpu->arch.bus_lock_detected = false;
>>
>> This is a fast path so I'm wondering if we can move bus_lock_detected
>> clearing somewhere else.
>
> Why even snapshot vmx->exit_reason.bus_lock_detected? I don't see any
> reason why vcpu_enter_guest() needs to handle the exit to userspace, e.g.
> it's just as easily handled in VMX code.

Because we want to handle the exit to userspace only in one place, i.e.,
after kvm_x86_ops.handle_exit(vcpu, exit_fastpath). Otherwise, we would
have to check vmx->exit_reason.bus_lock_detected in every other handler,
at least in those can preempt the bus lock VM-exit theoretically.

>>
>>> + }
>>> + }
>>> +
>>> vmx->loaded_vmcs->launched = 1;
>>> vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD);
>>>
>>> @@ -8060,6 +8088,9 @@ static __init int hardware_setup(void)
>>> kvm_tsc_scaling_ratio_frac_bits = 48;
>>> }
>>>
>>> + if (cpu_has_vmx_bus_lock_detection())
>>> + kvm_has_bus_lock_exit = true;
>>> +
>>> set_bit(0, vmx_vpid_bitmap); /* 0 is reserved for host */
>>>
>>> if (enable_ept)
>
> ...
>
>>> @@ -4990,6 +4996,12 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>>> kvm->arch.exception_payload_enabled = cap->args[0];
>>> r = 0;
>>> break;
>>> + case KVM_CAP_X86_BUS_LOCK_EXIT:
>>> + if (!kvm_has_bus_lock_exit)
>>> + return -EINVAL;
>>
>> ... because userspace can check for -EINVAL when enabling the cap. Or we
>> can return e.g. -EOPNOTSUPP here. I don't have a strong opinion on the matter..
>>
>>> + kvm->arch.bus_lock_exit = cap->args[0];
>
> Assuming we even want to make this per-VM, I think it'd make sense to make
> args[0] a bit mask, e.g. to provide "off" and "exit" (this behavior) while
> allowing for future modes, e.g. log-only.

Good idea, will do it in next version.

>>> + r = 0;
>>> + break;
>>> default:
>>> r = -EINVAL;
>>> break;
>>> @@ -7732,12 +7744,23 @@ static void post_kvm_run_save(struct kvm_vcpu *vcpu)

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