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    SubjectRe: [PATCH 16/39] irqdomain: Make normal and nomap irqdomains exclusive
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    Hello Marc,

    >> This patch is breaking the POWER9/POWER10 XIVE driver (these are not
    >> old PPC systems :) on machines sharing the same LSI HW IRQ. For instance,
    >> a linux KVM guest with a virtio-rng and a virtio-balloon device. In that
    >> case, Linux creates two distinct IRQ mappings which can lead to some
    >> unexpected behavior.
    >
    > Either the irq domain translates, or it doesn't. If the driver creates
    > a nomap domain, and yet expects some sort of translation to happen,
    > then the driver is fundamentally broken. And even without that: how do
    > you end-up with a single HW interrupt having two mappings?
    >
    >> A fix to go forward would be to change the XIVE IRQ domain to use a
    >> 'Tree' domain for reverse mapping and not the 'No Map' domain mapping.
    >> I will keep you updated for XIVE.
    >
    > I bet there is a bit more to it. From what you are saying above,
    > something rather ungodly is happening in the XIVE code.

    It's making progress.

    This change in irq_find_mapping() is what 'breaks' XIVE :

    + if (irq_domain_is_nomap(domain)) {
    + if (hwirq < domain->revmap_size) {
    + data = irq_domain_get_irq_data(domain, hwirq);
    + if (data && data->hwirq == hwirq)
    + return hwirq;
    + }
    +
    + return 0;


    With the introduction of IRQ_DOMAIN_FLAG_NO_MAP, the revmap_tree lookup
    is skipped and the previously mapped IRQ is not found. XIVE was relying
    on a side effect of irq_domain_set_mapping() which is not true anymore.

    I guess the easiest fix for 5.14 and 5.15 (in which was introduced MSI
    domains) is to change the XIVE IRQ domain to a domain tree. Since the HW
    can handle 1MB interrupts, this looks like a better choice for the driver.

    Thanks,

    C.

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