[PATCH v3] KVM: VMX: Remove redundant handling of bus lock vmexit

[PATCH v3] KVM: VMX: Remove redundant handling of bus lock vmexit

[Hao Xiang]

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. Unconditionally Set
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: Xiaoyao Li <xiaoyao.li@intel.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Hao Xiang <hao.xiang@linux.alibaba.com>
---
v1 -> v2: a little modifications of comments
v2 -> v3: addressed the review 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..7fb2a3a 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. Unconditionally set the flag here and leave the handling to
+	 * vmx_handle_exit().
+	 */
+	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)
-- 
1.8.3.1

Re: [PATCH v3] KVM: VMX: Remove redundant handling of bus lock vmexit

[Sean Christopherson]

On Fri, Oct 15, 2021, 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. Unconditionally Set
> 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: Xiaoyao Li <xiaoyao.li@intel.com>

Code review feedback generally doesn't warrant a Suggested-by.  The intent of
Suggested-by is to give credit to the idea/approach of a patch, so unless the
review feedback suggests a completely different, noting the input in the delta
(as you did below) is sufficient.  And then that way you don't need to juggle
the Suggested-by vs Co-developed-by for me.

> Co-developed-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

> Signed-off-by: Hao Xiang <hao.xiang@linux.alibaba.com>
> ---
> v1 -> v2: a little modifications of comments
> v2 -> v3: addressed the review 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..7fb2a3a 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. Unconditionally set the flag here and leave the handling to
> +	 * vmx_handle_exit().

+1 for "Unconditionally" instead of "Force".  Any objection to rewording the
second half slightly?

	/*
	 * Hardware may or may not set the BUS_LOCK_DETECTED flag on BUS_LOCK
	 * VM-Exits. Unconditionally set the flag here and let vmx_handle_exit()
	 * handle all flavors of bus-lock exits.
	 */

Not a big deal in this case, but in the future please give reviewers a chance to
respond and wait for discussion to settle before sending a new version, e.g. I
would happily have replied to Xiaoyao's suggestion in v2.

Re: [PATCH v3] KVM: VMX: Remove redundant handling of bus lock vmexit

[Paolo Bonzini]

On 15/10/21 13:59, 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. Unconditionally Set
> 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: Xiaoyao Li <xiaoyao.li@intel.com>
> Co-developed-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Hao Xiang <hao.xiang@linux.alibaba.com>
> ---
> v1 -> v2: a little modifications of comments
> v2 -> v3: addressed the review 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..7fb2a3a 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. Unconditionally set the flag here and leave the handling to
> +	 * vmx_handle_exit().
> +	 */
> +	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)
> 

Queued, with extra *-by tags removed.  Thanks,

Paolo