[PATCH RFC] KVM: nSVM: remove useless kvm_clear_*_queue

[PATCH RFC] KVM: nSVM: remove useless kvm_clear_*_queue

[Paolo Bonzini]

For an event to be in injected state when nested_svm_vmrun executes,
it must have come from exitintinfo when svm_complete_interrupts ran:

  vcpu_enter_guest
   static_call(kvm_x86_run) -> svm_vcpu_run
    svm_complete_interrupts
     // now the event went from "exitintinfo" to "injected"
   static_call(kvm_x86_handle_exit) -> handle_exit
    svm_invoke_exit_handler
      vmrun_interception
       nested_svm_vmrun

However, no event could have been in exitintinfo before a VMRUN
vmexit.  The code in svm.c is a bit more permissive than the one
in vmx.c:

        if (is_external_interrupt(svm->vmcb->control.exit_int_info) &&
            exit_code != SVM_EXIT_EXCP_BASE + PF_VECTOR &&
            exit_code != SVM_EXIT_NPF && exit_code != SVM_EXIT_TASK_SWITCH &&
            exit_code != SVM_EXIT_INTR && exit_code != SVM_EXIT_NMI)

but in any case, a VMRUN instruction would not even start to execute
during an attempted event delivery.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 arch/x86/kvm/svm/nested.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 61738ff8ef33..5e13357da21e 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -659,11 +659,6 @@ int nested_svm_vmrun(struct kvm_vcpu *vcpu)
 		goto out;
 	}
 
-
-	/* Clear internal status */
-	kvm_clear_exception_queue(vcpu);
-	kvm_clear_interrupt_queue(vcpu);
-
 	/*
 	 * Since vmcb01 is not in use, we can use it to store some of the L1
 	 * state.
-- 
2.27.0

Re: [PATCH RFC] KVM: nSVM: remove useless kvm_clear_*_queue

[Maxim Levitsky]

On Mon, 2021-08-02 at 08:56 -0400, Paolo Bonzini wrote:
> For an event to be in injected state when nested_svm_vmrun executes,
> it must have come from exitintinfo when svm_complete_interrupts ran:
> 
>   vcpu_enter_guest
>    static_call(kvm_x86_run) -> svm_vcpu_run
>     svm_complete_interrupts
>      // now the event went from "exitintinfo" to "injected"
>    static_call(kvm_x86_handle_exit) -> handle_exit
>     svm_invoke_exit_handler
>       vmrun_interception
>        nested_svm_vmrun
> 
> However, no event could have been in exitintinfo before a VMRUN
> vmexit.  The code in svm.c is a bit more permissive than the one
> in vmx.c:
> 
>         if (is_external_interrupt(svm->vmcb->control.exit_int_info) &&
>             exit_code != SVM_EXIT_EXCP_BASE + PF_VECTOR &&
>             exit_code != SVM_EXIT_NPF && exit_code != SVM_EXIT_TASK_SWITCH &&
>             exit_code != SVM_EXIT_INTR && exit_code != SVM_EXIT_NMI)
> 
> but in any case, a VMRUN instruction would not even start to execute
> during an attempted event delivery.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/svm/nested.c | 5 -----
>  1 file changed, 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 61738ff8ef33..5e13357da21e 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -659,11 +659,6 @@ int nested_svm_vmrun(struct kvm_vcpu *vcpu)
>  		goto out;
>  	}
>  
> -
> -	/* Clear internal status */
> -	kvm_clear_exception_queue(vcpu);
> -	kvm_clear_interrupt_queue(vcpu);
> -
>  	/*
>  	 * Since vmcb01 is not in use, we can use it to store some of the L1
>  	 * state.
100% agree.

As I say, Intel's architects weren't crazy enough to implement an VM IDT gate
(switch to a VM on interrupt....), and so indeed that this isn't possible.
They do have a task gate... thankfully it is legacy.

I would still keep a WARN_ON_ONCE here just in case.

Note that in theory one can force an injected/queued exception with KVM_SET_VCPU_EVENTS
on RIP that points to VMRUN instruction. Its user fault though.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
	Maxim Levitsky