Re: [PATCH 1/4 v5] KVM: nSVM: If VMRUN is single-stepped, queue the #DB intercept in nested_svm_vmexit()

[Paolo Bonzini <pbonzini@redhat.com>]

On 23/03/21 18:50, Krish Sadhukhan wrote:
> According to APM, the #DB intercept for a single-stepped VMRUN must happen
> after the completion of that instruction, when the guest does #VMEXIT to
> the host. However, in the current implementation of KVM, the #DB intercept
> for a single-stepped VMRUN happens after the completion of the instruction
> that follows the VMRUN instruction. When the #DB intercept handler is
> invoked, it shows the RIP of the instruction that follows VMRUN, instead of
> of VMRUN itself. This is an incorrect RIP as far as single-stepping VMRUN
> is concerned.
> 
> This patch fixes the problem by checking, in nested_svm_vmexit(), for the
> condition that the VMRUN instruction is being single-stepped and if so,
> queues the pending #DB intercept so that the #DB is accounted for before
> we execute L1's next instruction.
> 
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Krish Sadhukhan <krish.sadhukhan@oraacle.com>
> ---
>   arch/x86/kvm/svm/nested.c | 10 ++++++++++
>   1 file changed, 10 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 35891d9a1099..713ce5cfb0db 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -720,6 +720,16 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
>   	kvm_clear_exception_queue(&svm->vcpu);
>   	kvm_clear_interrupt_queue(&svm->vcpu);
>   
> +	/*
> +	 * If we are here following the completion of a VMRUN that
> +	 * is being single-stepped, queue the pending #DB intercept
> +	 * right now so that it an be accounted for before we execute
> +	 * L1's next instruction.
> +	 */
> +	if (unlikely(svm->vmcb->control.exit_code == SVM_EXIT_VMRUN &&
> +	    svm->vmcb->save.rflags & X86_EFLAGS_TF))
> +		kvm_queue_exception(&(svm->vcpu), DB_VECTOR);
> +
>   	return 0;
>   }

Wouldn't the exit code always be SVM_EXIT_VMRUN after the vmcb01/vmcb02 
split?  I can take care of adding a WARN_ON myself, but I wouldn't mind 
if you checked that my reasoning is true. :)

Paolo