Re: [PATCH v4 15/32] Revert "KVM: SVM: Use target APIC ID to complete x2AVIC IRQs when possible"

[Maxim Levitsky]

On Sat, 2022-10-01 at 00:58 +0000, Sean Christopherson wrote:
> Due to a likely mismerge of patches, KVM ended up with a superfluous
> commit to "enable" AVIC's fast path for x2AVIC mode.  Even worse, the
> superfluous commit has several bugs and creates a nasty local shadow
> variable.
> 
> Rather than fix the bugs piece-by-piece[*] to achieve the same end
> result, revert the patch wholesale.
> 
> Opportunistically add a comment documenting the x2AVIC dependencies.
> 
> This reverts commit 8c9e639da435874fb845c4d296ce55664071ea7a.
> 
> [*] https://lore.kernel.org/all/YxEP7ZBRIuFWhnYJ@xxxxxxxxxx
> 
> Fixes: 8c9e639da435 ("KVM: SVM: Use target APIC ID to complete x2AVIC IRQs when possible")
> Suggested-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/avic.c | 29 +++++++++++------------------
>  1 file changed, 11 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index e35e9363e7ff..605c36569ddf 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -378,7 +378,17 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
>  
>  		logid_index = cluster + __ffs(bitmap);
>  
> -		if (!apic_x2apic_mode(source)) {
> +		if (apic_x2apic_mode(source)) {
> +			/*
> +			 * For x2APIC, the logical APIC ID is a read-only value
> +			 * that is derived from the x2APIC ID, thus the x2APIC
> +			 * ID can be found by reversing the calculation (done
> +			 * above).  Note, bits 31:20 of the x2APIC ID are not
> +			 * propagated to the logical ID, but KVM limits the
> +			 * x2APIC ID limited to KVM_MAX_VCPU_IDS.
> +			 */
> +			l1_physical_id = logid_index;
> +		} else {
>  			u32 *avic_logical_id_table =
>  				page_address(kvm_svm->avic_logical_id_table_page);
>  
> @@ -393,23 +403,6 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
>  
>  			l1_physical_id = logid_entry &
>  					 AVIC_LOGICAL_ID_ENTRY_GUEST_PHYSICAL_ID_MASK;
> -		} else {
> -			/*
> -			 * For x2APIC logical mode, cannot leverage the index.
> -			 * Instead, calculate physical ID from logical ID in ICRH.
> -			 */
> -			int cluster = (icrh & 0xffff0000) >> 16;
> -			int apic = ffs(icrh & 0xffff) - 1;
> -
> -			/*
> -			 * If the x2APIC logical ID sub-field (i.e. icrh[15:0])
> -			 * contains anything but a single bit, we cannot use the
> -			 * fast path, because it is limited to a single vCPU.
> -			 */
> -			if (apic < 0 || icrh != (1 << apic))
> -				return -EINVAL;
> -
> -			l1_physical_id = (cluster << 4) + apic;
>  		}
>  	}
>  


Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky