Re: [RFC 2/2] KVM: VMX: Enable bus lock VM exit

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Xiaoyao Li <xiaoyao.li@intel.com> writes:

> 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.

Exiting to userspace would allow to implement custom 'throttling'
policies to mitigate the 'noisy neighbour' problem. The simplest would
be to just inject some sleep time.

-- 
Vitaly