Re: [PATCH 01/19] KVM: SVM: Process ICR on AVIC IPI delivery failure due to invalid target

[Maxim Levitsky]

On Wed, 2022-08-31 at 00:34 +0000, Sean Christopherson wrote:
> Emulate ICR writes on AVIC IPI failures due to invalid targets using the
> same logic as failures due to invalid types.  AVIC acceleration fails if
> _any_ of the targets are invalid
> , and crucially VM-Exits before sending
> IPIs to targets that _are_ valid.  In logical mode, the destination is a
> bitmap, i.e. a single IPI can target multiple logical IDs.  Doing nothing
> causes KVM to drop IPIs if at least one target is valid and at least one
> target is invalid.


This is a corner case, but it does makes sense to fix it.

Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky

> 
> Fixes: 18f40c53e10f ("svm: Add VMEXIT handlers for AVIC")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/avic.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 6919dee69f18..b1ade555e8d0 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -496,14 +496,18 @@ int avic_incomplete_ipi_interception(struct kvm_vcpu *vcpu)
>  	trace_kvm_avic_incomplete_ipi(vcpu->vcpu_id, icrh, icrl, id, index);
>  
>  	switch (id) {
> +	case AVIC_IPI_FAILURE_INVALID_TARGET:
>  	case AVIC_IPI_FAILURE_INVALID_INT_TYPE:
>  		/*
>  		 * Emulate IPIs that are not handled by AVIC hardware, which
> -		 * only virtualizes Fixed, Edge-Triggered INTRs.  The exit is
> -		 * a trap, e.g. ICR holds the correct value and RIP has been
> -		 * advanced, KVM is responsible only for emulating the IPI.
> -		 * Sadly, hardware may sometimes leave the BUSY flag set, in
> -		 * which case KVM needs to emulate the ICR write as well in
> +		 * only virtualizes Fixed, Edge-Triggered INTRs, and falls over
> +		 * if _any_ targets are invalid, e.g. if the logical mode mask
> +		 * is a superset of running vCPUs.
> +		 *
> +		 * The exit is a trap, e.g. ICR holds the correct value and RIP
> +		 * has been advanced, KVM is responsible only for emulating the
> +		 * IPI.  Sadly, hardware may sometimes leave the BUSY flag set,
> +		 * in which case KVM needs to emulate the ICR write as well in
>  		 * order to clear the BUSY flag.
>  		 */
>  		if (icrl & APIC_ICR_BUSY)
> @@ -519,8 +523,6 @@ int avic_incomplete_ipi_interception(struct kvm_vcpu *vcpu)
>  		 */
>  		avic_kick_target_vcpus(vcpu->kvm, apic, icrl, icrh, index);
>  		break;
> -	case AVIC_IPI_FAILURE_INVALID_TARGET:
> -		break;
>  	case AVIC_IPI_FAILURE_INVALID_BACKING_PAGE:
>  		WARN_ONCE(1, "Invalid backing page\n");
>  		break;