Re: [PATCH] um: Support SPARSE_IRQ

[David Gow <davidgow@google.com>]

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On Tue, 26 Aug 2025 at 05:20, Brian Norris <briannorris@chromium.org> wrote:
>
> From: Sinan Nalkaya <sardok@gmail.com>
>
> Motivation: IRQ KUnit tests are going to require CONFIG_SPARSE_IRQ [1] in
> order to:
> (a) reliably allocate additional (fake) IRQs and
> (b) ensure we can test managed affinity, which is only supported with
>     SPARSE_IRQ.
>
> It seems that the only thing necessary for ARCH=um is to tell the genirq
> core to skip over our preallocated NR_IRQS.
>
> Tested with:
>
>   $ ./tools/testing/kunit/kunit.py run
>   [...]
>   [13:55:58] Testing complete. Ran 676 tests: passed: 646, skipped: 30
>   [...]
>
> This comparse with pre-patch results:
>
>     Ran 672 tests: passed: 644, skipped: 28
>
> i.e., we no longer skip tests that 'depend on SPARSE_IRQ', and existing
> tests all pass.
>
> [1]
> [PATCH v2 4/6] genirq/test: Depend on SPARSE_IRQ
> https://lore.kernel.org/all/CABVgOSngoD0fh1WEkUCEwSdk0Joypo3dA_Y_SjW+K=nVDnZs3Q@mail.gmail.com/
>
> Signed-off-by: Sinan Nalkaya <sardok@gmail.com>
> [Brian: Adapted Sinan's patch; rewrote commit message]
> Signed-off-by: Brian Norris <briannorris@chromium.org>
> ---

Thanks -- this seems to work well here. I can confirm the KUnit tests
pass (including with 32-bit builds), and nothing I've tested has
failed so far.

Tested-by: David Gow <davidgow@google.com>

Cheers,
-- David

> This is adapted from Sinan's work at:
> [PATCH 1/1] um: Fix broken IRQs if SPARSE_IRQ is selected
> https://lore.kernel.org/all/1360193940-31504-1-git-send-email-sardok@gmail.com/
>
> Place any blame for errors on me of course.
>
> I'm not much of a UML developer, although I've been developing KUnit
> tests which default to running on ARCH=um.
>
> I also can't quite tell if MAY_HAVE_SPARSE_IRQ or SPARSE_IRQ is a better
> 'select' target. Almost every other architecture uses 'select
> SPARSE_IRQ', with the one exception of arch/csky. For my purposes, it's
> better to 'select SPARSE_IRQ', for consistency with other ARCH'es, and
> to make it easier for KUnit builds to get it. But I'm less sure if there
> are good reasons to want to make it user-(un)selectable.
>
>  arch/um/Kconfig      | 1 +
>  arch/um/kernel/irq.c | 7 +++++++
>  2 files changed, 8 insertions(+)
>
> diff --git a/arch/um/Kconfig b/arch/um/Kconfig
> index 9083bfdb7735..8161cc5ae6f7 100644
> --- a/arch/um/Kconfig
> +++ b/arch/um/Kconfig
> @@ -38,6 +38,7 @@ config UML
>         select HAVE_ARCH_TRACEHOOK
>         select HAVE_SYSCALL_TRACEPOINTS
>         select THREAD_INFO_IN_TASK
> +       select SPARSE_IRQ
>
>  config MMU
>         bool
> diff --git a/arch/um/kernel/irq.c b/arch/um/kernel/irq.c
> index 0dfaf96bb7da..d59a5a0f7fbf 100644
> --- a/arch/um/kernel/irq.c
> +++ b/arch/um/kernel/irq.c
> @@ -691,6 +691,13 @@ void __init init_IRQ(void)
>         os_setup_epoll();
>  }
>
> +#ifdef CONFIG_SPARSE_IRQ
> +int __init arch_probe_nr_irqs(void)
> +{
> +       return NR_IRQS;
> +}
> +#endif
> +
>  void sigchld_handler(int sig, struct siginfo *unused_si,
>                      struct uml_pt_regs *regs, void *mc)
>  {
> --
> 2.51.0.261.g7ce5a0a67e-goog
>

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