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Subject: Re: [RFC 2/2] KVM: VMX: Enable bus lock VM exit
To:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Chenyi Qiang <chenyi.qiang@intel.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>
References: <20200628085341.5107-1-chenyi.qiang@intel.com>
 <20200628085341.5107-3-chenyi.qiang@intel.com>
 <878sg3bo8b.fsf@vitty.brq.redhat.com>
From:   Xiaoyao Li <xiaoyao.li@intel.com>
Message-ID: <0159554d-82d5-b388-d289-a5375ca91323@intel.com>
Date:   Wed, 1 Jul 2020 17:32:53 +0800
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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.