Hi Mark,

On Fri, 19 Feb 2021 11:38:56 +0000,
Mark Rutland [off-list ref] wrote:

Hector's M1 support series [1] shows that some platforms have critical
interrupts wired to FIQ, and to support these platforms we need to support
handling FIQ exceptions. Other contemporary platforms don't use FIQ (since e.g.
this is usually routed to EL3), and as we never expect to take an FIQ, we have
the FIQ vector cause a panic.

Since the use of FIQ is a platform integration detail (which can differ across
bare-metal and virtualized environments), we need be able to explicitly opt-in
to handling FIQs while retaining the existing behaviour otherwise. This series
adds a new set_handle_fiq() hook so that the FIQ controller can do so, and
where no controller is registered the default handler will panic(). For
consistency the set_handle_irq() code is made to do the same.

The first couple of patches are from Marc's irq/drop-generic_irq_multi_handler
branch [2] on kernel.org, and clean up CONFIG_GENERIC_IRQ_MULTI_HANDLER usage.
The next four patches move arm64 over to a local set_handle_irq()
implementation, which is written to share code with a set_handle_fiq() function
in the last two patches. The only functional difference here is that if an IRQ
is somehow taken prior to set_handle_irq() the default handler will directly
panic() rather than the vector branching to NULL.

The penultimate patch is cherry-picked from the v2 M1 series, and as per
discussion there [3] will need a few additional fixups. I've included it for
now as the DAIF.IF alignment is necessary for the FIQ exception handling added
in the final patch.

The final patch adds the low-level FIQ exception handling and registration
mechanism atop the prior rework.

Thanks for putting this together. I have an extra patch on top of this
series[1] that prevents the kernel from catching fire if a FIQ fires
whilst running a guest. Nothing urgent, we can queue it at a later time.

Thanks,

	M.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=irq/fiq

-- 
Without deviation from the norm, progress is not possible.

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