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

From: Xiaoyao Li <xiaoyao.li@intel.com>
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>
Subject: Re: [RFC 2/2] KVM: VMX: Enable bus lock VM exit
Date: Wed, 1 Jul 2020 22:12:22 +0800	[thread overview]
Message-ID: <adad61e8-8252-0491-7feb-992a52c1b4f3@intel.com> (raw)
In-Reply-To: <87366bbe1y.fsf@vitty.brq.redhat.com>

On 7/1/2020 8:44 PM, Vitaly Kuznetsov wrote:
> 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.
> 

So you want an exit to userspace for every bus lock and leave it all to 
userspace. Yes, it's doable.

As you said, the exit_reason.bus_lock_detected case is the tricky one. 
We cannot do the similar to extend vcpu->run->exit_reason, this breaks 
ABI. Maybe we can extend the vcpu->run->flags to indicate bus lock 
detected for the other exit reason?