irq_build_affinity_masks() allocates improper affinity if num_possible_cpus() > num_present_cpus()?

[Dexuan Cui <decui@microsoft.com>]

Hi all,
I'm running a single-CPU Linux VM on Hyper-V. The Linux kernel is v5.9-rc7
and I have CONFIG_NR_CPUS=256.

The Hyper-V Host (Version 17763-10.0-1-0.1457) provides a guest firmware,
which always reports 128 Local APIC entries in the ACPI MADT table. Here
only the first Local APIC entry's "Processor Enabled" is 1 since this
Linux VM is configured to have only 1 CPU. This means: in the Linux kernel,
the "cpu_present_mask" and " cpu_online_mask " have only 1 CPU (i.e. CPU0),
while the "cpu_possible_mask" has 128 CPUs, and the "nr_cpu_ids" is 128.

I pass through an MSI-X-capable PCI device to the Linux VM (which has
only 1 virtual CPU), and the below code does *not* report any error
(i.e. pci_alloc_irq_vectors_affinity() returns 2, and request_irq()
returns 0), but the code does not work: the second MSI-X interrupt is not
happening while the first interrupt does work fine.

int nr_irqs = 2;
int i, nvec, irq;

nvec = pci_alloc_irq_vectors_affinity(pdev, nr_irqs, nr_irqs,
                PCI_IRQ_MSIX | PCI_IRQ_AFFINITY, NULL);

for (i = 0; i < nvec; i++) {
        irq = pci_irq_vector(pdev, i);
        err = request_irq(irq, test_intr, 0, "test_intr", &intr_cxt[i]);
}

It turns out that pci_alloc_irq_vectors_affinity() -> ... ->
irq_create_affinity_masks() allocates an improper affinity for the second
interrupt. The below printk() shows that the second interrupt's affinity is
1-64, but only CPU0 is present in the system! As a result, later,
request_irq() -> ... -> irq_startup() -> __irq_startup_managed() returns
IRQ_STARTUP_ABORT because cpumask_any_and(aff, cpu_online_mask) is 
empty (i.e. >= nr_cpu_ids), and irq_startup() *silently* fails (i.e. "return 0;"),
since __irq_startup() is only called for IRQ_STARTUP_MANAGED and
IRQ_STARTUP_NORMAL.

--- a/kernel/irq/affinity.c
+++ b/kernel/irq/affinity.c
@@ -484,6 +484,9 @@ struct irq_affinity_desc *
        for (i = affd->pre_vectors; i < nvecs - affd->post_vectors; i++)
                masks[i].is_managed = 1;

+       for (i = 0; i < nvecs; i++)
+               printk("i=%d, affi = %*pbl\n", i,
+                      cpumask_pr_args(&masks[i].mask));
        return masks;
 }

[   43.770477] i=0, affi = 0,65-127
[   43.770484] i=1, affi = 1-64

Though here the issue happens to a Linux VM on Hyper-V, I think the same
issue can also happen to a physical machine, if the physical machine also
uses a lot of static MADT entries, of which only the entries of the present
CPUs are marked to be "Processor Enabled == 1".

I think pci_alloc_irq_vectors_affinity() -> __pci_enable_msix_range() ->
irq_calc_affinity_vectors() -> cpumask_weight(cpu_possible_mask) should
use cpu_present_mask rather than cpu_possible_mask (), so here
irq_calc_affinity_vectors() would return 1, and
__pci_enable_msix_range() would immediately return -ENOSPC to avoid a
*silent* failure.

However, git-log shows that this 2018 commit intentionally changed the
cpu_present_mask to cpu_possible_mask:
84676c1f21e8 ("genirq/affinity: assign vectors to all possible CPUs")

so I'm not sure whether (and how?) we should address the *silent* failure.

BTW, here I use a single-CPU VM to simplify the discussion. Actually,
if the VM has n CPUs, with the above usage of
pci_alloc_irq_vectors_affinity() (which might seem incorrect, but my point is
that it's really not good to have a silent failure, which makes it a lot more 
difficult to figure out what goes wrong), it looks only the first n MSI-X interrupts
can work, and the (n+1)'th MSI-X interrupt can not work due to the allocated
improper affinity.

According to my tests, if we need n+1 MSI-X interrupts in such a VM that
has n CPUs, it looks we have 2 options (the second should be better):

1. Do not use the PCI_IRQ_AFFINITY flag, i.e.
        pci_alloc_irq_vectors_affinity(pdev, n+1, n+1, PCI_IRQ_MSIX, NULL);

2. Use the PCI_IRQ_AFFINITY flag, and pass a struct irq_affinity affd,
which tells the API that we don't care about the first interrupt's affinity:

        struct irq_affinity affd = {
                .pre_vectors = 1,
				...
        };

        pci_alloc_irq_vectors_affinity(pdev, n+1, n+1,
                PCI_IRQ_MSIX | PCI_IRQ_AFFINITY, &affd);

PS, irq_create_affinity_masks() is complicated. Let me know if you're
interested to know how it allocates the invalid affinity "1-64" for the
second MSI-X interrupt.

PS2, the latest Hyper-V provides only one ACPI MADT entry to a 1-CPU VM,
so the issue described above can not reproduce there.

Thanks,
-- Dexuan