linux-kernel - Re: [PATCH v2] KVM: VMX: Remove redundant handling of bus lock vmexit

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Date:   Fri, 15 Oct 2021 15:28:13 +0800
From:   Hao Xiang <hao.xiang@...ux.alibaba.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>, kvm@...r.kernel.org
Cc:     shannon.zhao@...ux.alibaba.com, pbonzini@...hat.com,
        linux-kernel@...r.kernel.org, seanjc@...gle.com
Subject: Re: [PATCH v2] KVM: VMX: Remove redundant handling of bus lock vmexit



On 2021/10/15 14:46, Xiaoyao Li wrote:
> On 10/15/2021 11:57 AM, Hao Xiang wrote:
>> Hardware may or may not set exit_reason.bus_lock_detected on BUS_LOCK
>> VM-Exits. Dealing with KVM_RUN_X86_BUS_LOCK in handle_bus_lock_vmexit
>> could be redundant when exit_reason.basic is EXIT_REASON_BUS_LOCK.
>>
>> We can remove redundant handling of bus lock vmexit. Force Setting
>> exit_reason.bus_lock_detected in handle_bus_lock_vmexit(), and deal with
>> KVM_RUN_X86_BUS_LOCK only in vmx_handle_exit().
>>
>> Suggested-by: Sean Christopherson <seanjc@...gle.com>
>> Signed-off-by: Hao Xiang <hao.xiang@...ux.alibaba.com>
>> ---
>> v1 -> v2: a little modifications of comments
>>
>>   arch/x86/kvm/vmx/vmx.c | 15 +++++++++------
>>   1 file changed, 9 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 116b089..22be02e 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -5562,9 +5562,13 @@ static int handle_encls(struct kvm_vcpu *vcpu)
>>   static int handle_bus_lock_vmexit(struct kvm_vcpu *vcpu)
>>   {
>> -    vcpu->run->exit_reason = KVM_EXIT_X86_BUS_LOCK;
>> -    vcpu->run->flags |= KVM_RUN_X86_BUS_LOCK;
>> -    return 0;
>> +    /*
>> +     * Hardware may or may not set the BUS_LOCK_DETECTED flag on 
>> BUS_LOCK
>> +     * VM-Exits, force setting the flag 
> 
>>          so that the logic in vmx_handle_exit()
>> +     * doesn't have to handle the flag and the basic exit reason.
> 
> The code itself looks good.
> 
> But the comment is partly incorrect. Now we rely only on the 
> bus_lock_detected flag so vmx_handle_exit() must check the flag.
>Thank you for you advice. Is the following comment ok?
/*
  * Hardware may or may not set the BUS_LOCK_DETECTED flag on BUS_LOCK
  * VM-Exits. Forcely set the flag here, then check the flag in
  * vmx_handle_exit() and handle the bus lock vmexit if the flag is
  * set.
  */

>> +     */
>> +    to_vmx(vcpu)->exit_reason.bus_lock_detected = true;
>> +    return 1;
>>   }
>>   /*
>> @@ -6051,9 +6055,8 @@ static int vmx_handle_exit(struct kvm_vcpu 
>> *vcpu, fastpath_t exit_fastpath)
>>       int ret = __vmx_handle_exit(vcpu, exit_fastpath);
>>       /*
>> -     * Even when current exit reason is handled by KVM internally, we
>> -     * still need to exit to user space when bus lock detected to inform
>> -     * that there is a bus lock in guest.
>> +     * Exit to user space when bus lock detected to inform that there is
>> +     * a bus lock in guest.
>>        */
>>       if (to_vmx(vcpu)->exit_reason.bus_lock_detected) {
>>           if (ret > 0)
>>