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Date:   Fri, 24 Jun 2022 18:41:38 +0200
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To:     Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org
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        jon.grimm@amd.com, wei.huang2@amd.com, terry.bowman@amd.com
References: <20220519102709.24125-1-suravee.suthikulpanit@amd.com>
 <20220519102709.24125-16-suravee.suthikulpanit@amd.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v6 15/17] KVM: SVM: Use target APIC ID to complete x2AVIC
 IRQs when possible
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On 5/19/22 12:27, Suravee Suthikulpanit wrote:
> +			 * If the x2APIC logical ID sub-field (i.e. icrh[15:0]) contains zero
> +			 * or more than 1 bits, we cannot match just one vcpu to kick for
> +			 * fast path.
> +			 */
> +			if (!first || (first != last))
> +				return -EINVAL;
> +
> +			apic = first - 1;
> +			if ((apic < 0) || (apic > 15) || (cluster >= 0xfffff))
> +				return -EINVAL;

Neither of these is possible: first == 0 has been cheked above, and
ffs(icrh & 0xffff) cannot exceed 15.  Likewise, cluster is actually
limited to 16 bits, not 20.

Plus, C is not Pascal so no parentheses. :)

Putting everything together, it can be simplified to this:

+                       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;


> +			apic_id = (cluster << 4) + apic;