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iommu/hyper-v: Don't assume cpu_possible_mask is dense

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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 <mhklinux@outlook.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241003035333.49261-4-mhklinux@outlook.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Message-ID: <20241003035333.49261-4-mhklinux@outlook.com>
This commit is contained in:
Michael Kelley 2024-10-02 20:53:31 -07:00 committed by Wei Liu
parent ef81fa8c53
commit 13546de234
1 changed files with 2 additions and 2 deletions
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4
drivers/iommu/hyperv-iommu.c
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@ -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;