linux-kernel - [PATCH 3/5] iommu/hyper-v: Don't assume cpu_possible

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Message-Id: <20241003035333.49261-4-mhklinux@outlook.com>
Date: Wed,  2 Oct 2024 20:53:31 -0700
From: mhkelley58@...il.com
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Subject: [PATCH 3/5] iommu/hyper-v: Don't assume cpu_possible_mask is dense

From: Michael Kelley <mhklinux@...look.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/SN6PR02MB4157210CC36B2593F8572E5ED4692@SN6PR02MB4157.namprd02.prod.outlook.com/

Signed-off-by: Michael Kelley <mhklinux@...look.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