Re: [PATCH v6 15/17] KVM: SVM: Use target APIC ID to complete x2AVIC IRQs when possible

Paolo Bonzini
2022-06-24T16:41:48Z

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;