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    SubjectRe: Xen regression, Was: [PATCH] x86/irq: Probe for PIC presence before allocating descs for legacy IRQs
    On Tue, 12 Apr 2016, Boris Ostrovsky wrote:
    > On 04/11/2016 10:08 PM, Stefano Stabellini wrote:
    > > Hi all,
    > >
    > > Unfortunately this patch (now commit
    > > 8c058b0b9c34d8c8d7912880956543769323e2d8) causes a regression on Xen
    > > when running on top of QEMU: the number of PIT irqs get set to 0 by
    > > probe_8259A but actually there are 16.
    > >
    > > Any suggestions on how to fix this?
    > >
    > > 1) we could revert 8c058b0b9c34d8c8d7912880956543769323e2d8
    > > 2) we could introduce an 'if (!xen_domain())' in probe_8259A
    > > 3) suggestions welcome
    >
    > Stefano, do you have b4ff8389ed14b849354b59ce9b360bdefcdbf99c ?
    >
    > It was supposed to fix this problem for Xen. However, I just noticed that
    > arch/arm64/include/asm/irq.h makes nr_legacy_irqs() return 0 (unlike
    > arch/arm/include/asm/irq.h). Could that be the problem?

    I have b4ff8389ed14b849354b59ce9b360bdefcdbf99c but it doesn't fix the
    issue for me.

    Is the idea of your patch that xen_allocate_irq_gsi will allocate the
    descriptor dynamically instead? If so, it doesn't work because it
    doesn't get called for irq 14:

    piix_init_one -> ata_pci_sff_activate_host -> devm_request_irq ->
    devm_request_threaded_irq-> request_threaded_irq -> irq_to_desc(14) ->
    -EVAIL

    If you look at pci_xen_initial_domain, the loop:

    for (irq = 0; irq < nr_legacy_irqs(); irq++) {

    won't work anymore because by the time is called, nr_legacy_irqs()
    already returns 0, because it has been changed by probe_8259A().

    We also need the following patch:

    ---

    xen/x86: actually allocate legacy interrupts on PV guests

    b4ff8389ed14 is incomplete: relies on nr_legacy_irqs() to get the number
    of legacy interrupts when actually nr_legacy_irqs() returns 0 after
    probe_8259A(). Use NR_IRQS_LEGACY instead.

    Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>

    diff --git a/arch/x86/pci/xen.c b/arch/x86/pci/xen.c
    index beac4df..6db0060 100644
    --- a/arch/x86/pci/xen.c
    +++ b/arch/x86/pci/xen.c
    @@ -492,7 +492,7 @@ int __init pci_xen_initial_domain(void)
    __acpi_register_gsi = acpi_register_gsi_xen;
    __acpi_unregister_gsi = NULL;
    /* Pre-allocate legacy irqs */
    - for (irq = 0; irq < nr_legacy_irqs(); irq++) {
    + for (irq = 0; irq < NR_IRQS_LEGACY; irq++) {
    int trigger, polarity;

    if (acpi_get_override_irq(irq, &trigger, &polarity) == -1)
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