RE: [PATCH V4 2/3] HYPERV/IOMMU: Add Hyper-V stub IOMMU driver

From: Michael Kelley
Date: Thu Feb 21 2019 - 12:27:46 EST


From: lantianyu1986@xxxxxxxxx <lantianyu1986@xxxxxxxxx> Sent: Monday, February 11, 2019 6:20 AM
> + /*
> + * Hyper-V doesn't provide irq remapping function for
> + * IO-APIC and so IO-APIC only accepts 8-bit APIC ID.
> + * Cpu's APIC ID is read from ACPI MADT table and APIC IDs
> + * in the MADT table on Hyper-v are sorted monotonic increasingly.
> + * APIC ID reflects cpu topology. There maybe some APIC ID
> + * gaps when cpu number in a socket is not power of two. Prepare
> + * max cpu affinity for IOAPIC irqs. Scan cpu 0-255 and set cpu
> + * into ioapic_max_cpumask if its APIC ID is less than 256.
> + */
> + for (i = min_t(unsigned int, num_possible_cpus(), 255); i >= 0; i--)

The above isn't quite right. For example, if num_possible_cpus() is 8,
then the loop will be executed 9 times, for values 8 down through 0.
It should be executed for values 7 down through 0.

> + if (cpu_physical_id(i) < 256)
> + cpumask_set_cpu(i, &ioapic_max_cpumask);
> +
> + return 0;
> +}

Michael