linux-kernel - Re: [PATCH] KVM: VMX: Check if bus lock vmexit was preempted

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Date:   Wed, 22 Sep 2021 12:02:42 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Hao Xiang <hao.xiang@...ux.alibaba.com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, chenyi.qiang@...el.com,
        sean.j.christopherson@...el.com, shannon.zhao@...ux.alibaba.com
Subject: Re: [PATCH] KVM: VMX: Check if bus lock vmexit was preempted

On 18/09/21 13:30, Hao Xiang wrote:
> exit_reason.bus_lock_detected is not only set when bus lock VM exit
> was preempted, in fact, this bit is always set if bus locks are
> detected no matter what the exit_reason.basic is.
> 
> So the bus_lock_vmexit handling in vmx_handle_exit should be duplicated
> when exit_reason.basic is EXIT_REASON_BUS_LOCK(74). We can avoid it by
> checking if bus lock vmexit was preempted in vmx_handle_exit.

I don't understand, does this mean that bus_lock_detected=1 if 
basic=EXIT_REASON_BUS_LOCK?  If so, can we instead replace the contents 
of handle_bus_lock_vmexit with

	/* Do nothing and let vmx_handle_exit exit to userspace.  */
	WARN_ON(!to_vmx(vcpu)->exit_reason.bus_lock_detected);
	return 0;

?

That would be doable only if this is architectural behavior and not a 
processor erratum, of course.

Thanks,

Paolo

> Signed-off-by: Hao Xiang <hao.xiang@...ux.alibaba.com>
> ---
>   arch/x86/kvm/vmx/vmx.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 0c2c0d5..5ddf1df 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6054,7 +6054,8 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t exit_fastpath)
>   	 * still need to 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 (to_vmx(vcpu)->exit_reason.bus_lock_detected &&
> +			to_vmx(vcpu)->exit_reason.basic != EXIT_REASON_BUS_LOCK) {
>   		if (ret > 0)
>   			vcpu->run->exit_reason = KVM_EXIT_X86_BUS_LOCK;
>   
>