linux-kernel - Re: [PATCH v5 27/33] KVM: SVM: Require logical ID to be power-of-2 for AVIC entry

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Message-ID: <86ad9834f8de2b2babb2bb6839d45288432f62de.camel@redhat.com>
Date:   Sun, 08 Jan 2023 17:20:56 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alejandro Jimenez <alejandro.j.jimenez@...cle.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Li RongQing <lirongqing@...du.com>,
        Greg Edwards <gedwards@....com>
Subject: Re: [PATCH v5 27/33] KVM: SVM: Require logical ID to be power-of-2
 for AVIC entry

On Fri, 2023-01-06 at 01:13 +0000, Sean Christopherson wrote:
> Do not modify AVIC's logical ID table if the logical ID portion of the
> LDR is not a power-of-2, i.e. if the LDR has multiple bits set.  Taking
> only the first bit means that KVM will fail to match MDAs that intersect
> with "higher" bits in the "ID"
> 
> The "ID" acts as a bitmap, but is referred to as an ID because there's an
> implicit, unenforced "requirement" that software only set one bit.  This
> edge case is arguably out-of-spec behavior, but KVM cleanly handles it
> in all other cases, e.g. the optimized logical map (and AVIC!) is also
> disabled in this scenario.
> 
> Refactor the code to consolidate the checks, and so that the code looks
> more like avic_kick_target_vcpus_fast().


Very minor nitpick about the subject, it feels like it just changes something
minor but it pretty much rewrites the avic_get_logical_id_entry.

When I bisected for the bug, this did confuse me a bit.

I don't have a good idea on how to call this patch so I won't object to the current
subject to stay as well.

Best regards,
	Maxim Levitsky


> 
> Fixes: 18f40c53e10f ("svm: Add VMEXIT handlers for AVIC")
> Cc: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
> Cc: Maxim Levitsky <mlevitsk@...hat.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/svm/avic.c | 30 +++++++++++++++---------------
>  1 file changed, 15 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 5b33f91b701c..eb979695e7d8 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -513,26 +513,26 @@ unsigned long avic_vcpu_get_apicv_inhibit_reasons(struct kvm_vcpu *vcpu)
>  static u32 *avic_get_logical_id_entry(struct kvm_vcpu *vcpu, u32 ldr, bool flat)
>  {
>  	struct kvm_svm *kvm_svm = to_kvm_svm(vcpu->kvm);
> -	int index;
>  	u32 *logical_apic_id_table;
> -	int dlid = GET_APIC_LOGICAL_ID(ldr);
> +	u32 cluster, index;
>  
> -	if (!dlid)
> -		return NULL;
> +	ldr = GET_APIC_LOGICAL_ID(ldr);
>  
> -	if (flat) { /* flat */
> -		index = ffs(dlid) - 1;
> -		if (index > 7)
> +	if (flat) {
> +		cluster = 0;
> +	} else {
> +		cluster = (ldr >> 4);
> +		if (cluster >= 0xf)
>  			return NULL;
> -	} else { /* cluster */
> -		int cluster = (dlid & 0xf0) >> 4;
> -		int apic = ffs(dlid & 0x0f) - 1;
> -
> -		if ((apic < 0) || (apic > 7) ||
> -		    (cluster >= 0xf))
> -			return NULL;
> -		index = (cluster << 2) + apic;
> +		ldr &= 0xf;
>  	}
> +	if (!ldr || !is_power_of_2(ldr))
> +		return NULL;
> +
> +	index = __ffs(ldr);
> +	if (WARN_ON_ONCE(index > 7))
> +		return NULL;
> +	index += (cluster << 2);
>  
>  	logical_apic_id_table = (u32 *) page_address(kvm_svm->avic_logical_id_table_page);
>  

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

Best regards,
	Maxim Levitsky