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

JIRA: https://issues.redhat.com/browse/RHEL-78704
Upstream Status: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

commit 4f6b64f
Author: Michael Kelley <mhklinux@outlook.com>
Date:   Wed Oct 2 20:53:31 2024 -0700

    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 <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>

(cherry picked from commit 4f6b64f)
Signed-off-by: Jerry Snitselaar <jsnitsel@redhat.com>

Author: snits

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;