From 4f6b64f3d3d96fb3796614362c64a4b73ddf3f7a Mon Sep 17 00:00:00 2001 From: Michael Kelley Date: Wed, 2 Oct 2024 20:53:31 -0700 Subject: [PATCH] iommu/hyper-v: Don't assume cpu_possible_mask is dense 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/SN6PR02MB4157210CC36B2593F8572E5ED4692@SN6PR02MB4157.namprd02.prod.outlook.com/ Signed-off-by: Michael Kelley Acked-by: Peter Zijlstra (Intel) Link: https://lore.kernel.org/r/20241003035333.49261-4-mhklinux@outlook.com Signed-off-by: Wei Liu Message-ID: <20241003035333.49261-4-mhklinux@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 8a5c17b973100..2a86aa5d54c68 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;