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SubjectRe: [RFC 2/2] KVM: VMX: Enable bus lock VM exit
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On 7/1/2020 5:04 PM, Vitaly Kuznetsov wrote:
> Chenyi Qiang <chenyi.qiang@intel.com> writes:
[...]
>> static const int kvm_vmx_max_exit_handlers =
>> @@ -6830,6 +6838,13 @@ 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 (vmx->exit_reason.bus_lock_detected)
>> + handle_bus_lock(vcpu);
>
> In case the ultimate goal is to have an exit to userspace on bus lock,

I don't think we will need an exit to userspace on bus lock. See below.

> the two ways to reach handle_bus_lock() are very different: in case
> we're handling EXIT_REASON_BUS_LOCK we can easily drop to userspace by
> returning 0 but what are we going to do in case of
> exit_reason.bus_lock_detected? The 'higher priority VM exit' may require
> exit to userspace too. So what's the plan? Maybe we can ignore the case
> when we're exiting to userspace for some other reason as this is slow
> already and force the exit otherwise?

> And should we actually introduce
> the KVM_EXIT_BUS_LOCK and a capability to enable it here?
>

Introducing KVM_EXIT_BUS_LOCK maybe help nothing. No matter
EXIT_REASON_BUS_LOCK or exit_reason.bus_lock_detected, the bus lock has
already happened. Exit to userspace cannot prevent bus lock, so what
userspace can do is recording and counting as what this patch does in
vcpu->stat.bus_locks.

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