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    SubjectRe: [PATCH] irqdomain: fix regression for Xen guests
    On Thu, Feb 23, 2023 at 09:38:00AM +0100, Juergen Gross wrote:
    > The recent switch to per-domain locking caused a NULL dereference in
    > irq_domain_create_hierarchy(), as Xen code is calling
    > msi_create_irq_domain() with a NULL parent pointer.

    This was unexpected. Apparently the Intel VMD driver also does this (see
    vmd_enable_domain()).

    > Fix that by testing parent to be set before dereferencing it. For a
    > non-existing parent the irq-domain's root will stay to point to
    > itself.

    An alternative could be to have msi_create_irq_domain() handle the
    no-parent case explicitly, but this looks the right fix for now (e.g. as
    irq_domain_create_tree() does not take a flag parameter).

    > Fixes: 9dbb8e3452ab ("irqdomain: Switch to per-domain locking")
    > Signed-off-by: Juergen Gross <jgross@suse.com>
    > ---
    > kernel/irq/irqdomain.c | 3 ++-
    > 1 file changed, 2 insertions(+), 1 deletion(-)
    >
    > diff --git a/kernel/irq/irqdomain.c b/kernel/irq/irqdomain.c
    > index aa5b7eeeceb8..6522dfb2e49c 100644
    > --- a/kernel/irq/irqdomain.c
    > +++ b/kernel/irq/irqdomain.c
    > @@ -1172,7 +1172,8 @@ struct irq_domain *irq_domain_create_hierarchy(struct irq_domain *parent,
    > domain = __irq_domain_create(fwnode, 0, ~0, 0, ops, host_data);
    >
    > if (domain) {
    > - domain->root = parent->root;
    > + if (parent)
    > + domain->root = parent->root;
    > domain->parent = parent;
    > domain->flags |= flags;

    Reviewed-by: Johan Hovold <johan+linaro@kernel.org>

    Johan

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