From: Michael Kelley <mhkli...@outlook.com> Current code gets the APIC IDs for CPUs numbered 255 and lower. This code assumes cpu_possible_mask is dense, which is not true in the general case per [1]. If cpu_possible_mask contains holes, num_possible_cpus() is less than nr_cpu_ids, so some CPUs might get skipped. Furthermore, getting the APIC ID of a CPU that isn't in cpu_possible_mask is invalid.
However, the configurations that Hyper-V provides to guest VMs on x86 hardware, in combination with how x86 code assigns Linux CPU numbers, *does* always produce a dense cpu_possible_mask. So the dense assumption is not currently causing failures. But for robustness against future changes in how cpu_possible_mask is populated, update the code to no longer assume dense. The correct approach is to determine the range to scan based on nr_cpu_ids, and skip any CPUs that are not in the cpu_possible_mask. [1] https://lore.kernel.org/lkml/sn6pr02mb4157210cc36b2593f8572e5ed4...@sn6pr02mb4157.namprd02.prod.outlook.com/ Signed-off-by: Michael Kelley <mhkli...@outlook.com> --- drivers/iommu/hyperv-iommu.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/drivers/iommu/hyperv-iommu.c b/drivers/iommu/hyperv-iommu.c index 8a5c17b97310..2a86aa5d54c6 100644 --- a/drivers/iommu/hyperv-iommu.c +++ b/drivers/iommu/hyperv-iommu.c @@ -164,8 +164,8 @@ static int __init hyperv_prepare_irq_remapping(void) * 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() - 1, 255); i >= 0; i--) - if (cpu_physical_id(i) < 256) + for (i = min_t(unsigned int, nr_cpu_ids - 1, 255); i >= 0; i--) + if (cpu_possible(i) && cpu_physical_id(i) < 256) cpumask_set_cpu(i, &ioapic_max_cpumask); return 0; -- 2.25.1
