Re: [PATCH v2 02/11] KVM: nSVM: clean up the copying of V_INTR bits from vmcb02 to vmcb12

[Sean Christopherson <seanjc@google.com>]

On Tue, Nov 29, 2022, Maxim Levitsky wrote:
> the V_IRQ and v_TPR bits don't exist when virtual interrupt
> masking is not enabled, therefore the KVM should not copy these
> bits regardless of V_IRQ intercept.

Hmm, the APM disagrees:

 The APIC's TPR always controls the task priority for physical interrupts, and the
 V_TPR always controls virtual interrupts.

   While running a guest with V_INTR_MASKING cleared to 0:
     • Writes to CR8 affect both the APIC's TPR and the V_TPR register.


 ...

 The three VMCB fields V_IRQ, V_INTR_PRIO, and V_INTR_VECTOR indicate whether there
 is a virtual interrupt pending, and, if so, what its vector number and priority are.

IIUC, V_INTR_MASKING_MASK is mostly about EFLAGS.IF, with a small side effect on
TPR.  E.g. a VMM could pend a V_IRQ but clear V_INTR_MASKING and expect the guest
to take the V_IRQ.  At least, that's my reading of things.

> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
>  arch/x86/kvm/svm/nested.c | 23 ++++++++---------------
>  1 file changed, 8 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 37af0338da7c32..aad3145b2f62fe 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -412,24 +412,17 @@ void nested_copy_vmcb_save_to_cache(struct vcpu_svm *svm,
>   */
>  void nested_sync_control_from_vmcb02(struct vcpu_svm *svm)
>  {
> -	u32 mask;
> +	u32 mask = 0;
>  	svm->nested.ctl.event_inj      = svm->vmcb->control.event_inj;
>  	svm->nested.ctl.event_inj_err  = svm->vmcb->control.event_inj_err;
>  
> -	/* Only a few fields of int_ctl are written by the processor.  */
> -	mask = V_IRQ_MASK | V_TPR_MASK;
> -	if (!(svm->nested.ctl.int_ctl & V_INTR_MASKING_MASK) &&
> -	    svm_is_intercept(svm, INTERCEPT_VINTR)) {
> -		/*
> -		 * In order to request an interrupt window, L0 is usurping
> -		 * svm->vmcb->control.int_ctl and possibly setting V_IRQ
> -		 * even if it was clear in L1's VMCB.  Restoring it would be
> -		 * wrong.  However, in this case V_IRQ will remain true until
> -		 * interrupt_window_interception calls svm_clear_vintr and
> -		 * restores int_ctl.  We can just leave it aside.
> -		 */
> -		mask &= ~V_IRQ_MASK;
> -	}
> +	/*
> +	 * Only a few fields of int_ctl are written by the processor.
> +	 * Copy back only the bits that are passed through to the L2.

Just "L2", not "the L2".

> +	 */
> +

Unnecessary newline.

> +	if (svm->nested.ctl.int_ctl & V_INTR_MASKING_MASK)
> +		mask = V_IRQ_MASK | V_TPR_MASK;
>  
>  	if (nested_vgif_enabled(svm))
>  		mask |= V_GIF_MASK;
> -- 
> 2.26.3
>