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

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Message-ID: <5eb45c93-38aa-cea8-a5c8-cf786c193342@intel.com>
Date:   Fri, 15 Oct 2021 14:46:11 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Hao Xiang <hao.xiang@...ux.alibaba.com>, kvm@...r.kernel.org
Cc:     shannon.zhao@...ux.alibaba.com, pbonzini@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: VMX: Remove redundant handling of bus lock vmexit

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.

> +	 */
> +	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)
>