Re: [PATCH] KVM: SVM: Fix x2apic logical cluster mode decoding and sanity check

[Sean Christopherson]

On Mon, Sep 12, 2022, Suravee Suthikulpanit wrote:
> When sending IPI in the X2APIC logical cluster mode, the destination
> APIC ID is encoded as:
> 
>   * Cluster ID = ICRH[31:16]
>   * Logical ID = ICRH[15:0]
> 
> Current logic incorrectly decode the ICRH, which causes VM running
> with x2AVIC support to fail to boot. Therefore, fix the decoding logic.

There are already patches pending[1][2] for the x2AVIC bugs.  

[1] https://lore.kernel.org/all/20220903002254.2411750-9-seanjc@xxxxxxxxxx
[2] https://lore.kernel.org/all/20220903002254.2411750-10-seanjc@xxxxxxxxxx

> 
> The commit 603ccef42ce9 ("KVM: x86: SVM: fix avic_kick_target_vcpus_fast")
> also added a check for multiple logical destinations before using
> the fast-path. However, the same logic is already existed prior to
> the commit. Therefore, remove redundant checking logic.
> 
> Fixes: 603ccef42ce9 ("KVM: x86: SVM: fix avic_kick_target_vcpus_fast")
> Cc: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
> ---
>  arch/x86/kvm/svm/avic.c | 19 ++++---------------
>  1 file changed, 4 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 6919dee69f18..45ab49d1f0b8 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -378,8 +378,8 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
>  
>  		if (apic_x2apic_mode(source)) {
>  			/* 16 bit dest mask, 16 bit cluster id */
> -			bitmap = dest & 0xFFFF0000;
> -			cluster = (dest >> 16) << 4;
> +			bitmap = dest & 0xffff;
> +			cluster = (dest & 0xffff0000) >> 16;
>  		} else if (kvm_lapic_get_reg(source, APIC_DFR) == APIC_DFR_FLAT) {
>  			/* 8 bit dest mask*/
>  			bitmap = dest;
> @@ -387,7 +387,7 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
>  		} else {
>  			/* 4 bit desk mask, 4 bit cluster id */
>  			bitmap = dest & 0xF;
> -			cluster = (dest >> 4) << 2;
> +			cluster = (dest & 0xf0) >> 4;

This is wrong and unrelated.  The cluster needs to be shifted back by 2, i.e. multiplied
by 4, to leap over each cluster of 4 IDs.

>  		}
>  
>  		if (unlikely(!bitmap))
> @@ -420,18 +420,7 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
>  			 * 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;
> +			l1_physical_id = (cluster << 4) + (ffs(bitmap) - 1);
>  		}
>  	}
>  
> -- 
> 2.34.1
>