[PATCH 0/4] genirq/msi: Fix device MSI prepare/alloc sequencing

Marc Zyngier
2025-05-11T16:35:26Z

While reviewing (and debugging) the GICv5 code, it became obvious that
there was something pretty fishy about the way MSI allocation was
taking place in respect to the msi_prepare() callback.

There is a strong (though not 100% explicit) requirement that the
.msi_prepare() callback must take place exactly once during the lifetime
of an MSI domain, before any MSI is allocated. Sadly, that's not how the
core code behaves, leading to all sorts of odd bad issues with MBIGEN or
the GICv5 IWB.

The fix is both simple and surprisingly convoluted. The first course
of action would be to hoist the "prepare" action before we allocate
any MSI, adding new tracking for the msi_alloc_info_t structure.

But this unveils another issue that the core code has been papering
over, which is that there is no pendant to the .msi_prepare() callback
that would be called on irqdomain destruction, and that at least the
ITS code has been using some crap heuristics to work around it.

So this needs to be addressed first, and that's the point of the first
two patches, introducing and implementing the .msi_teardown() callback.

The last patch remove a gross hack in the ITS msi-parent code, which
was papering over the broken use of the "prepare" callback.

This has been tested on a GICv4 system with MBIGEN, patches on top of
6.15-rc5.

Marc Zyngier (4):
genirq/msi: Add .msi_teardown() callback as the reverse of
.msi_prepare()
irqchip/gic-v3-its: Implement .msi_teardown() callback
genirq/msi: Move prepare() call to per-device allocation
irqchip/gic-v3-its: Use allocation size from the prepare call

drivers/irqchip/irq-gic-v3-its-msi-parent.c | 29 ++++-------
drivers/irqchip/irq-gic-v3-its.c | 56 +++++++++++++--------
include/linux/msi.h | 12 ++++-
kernel/irq/msi.c | 51 +++++++++++++++++--
4 files changed, 101 insertions(+), 47 deletions(-)

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2.39.2