Re: [RFC PATCH] arm_mpam: remove sanity check of accessibility when error interrupt is SPI on SMT platforms
From: Ben Horgan <ben.horgan@arm.com>
Date: 2026-07-28 15:48:57
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linux-acpi, lkml
Hi Yadong, On 7/20/26 08:24, Yadong Qi wrote:
On SMT platforms, an L2 cache MSC is typically shared between sibling threads. Per the MPAM ACPI spec (DEN0065B), the MSC's linked device should be set to the processor container of those siblings, so the kernel derives msc->accessibility as just the sibling CPUs. Per the MPAM spec (IHI0099B), when an MSC is not integrated into a PE, SPI/LPI is the recommended error interrupt. SPIs are not per-CPU, and a sanity check in mpam_msc_setup_error_irq() requires accessibility == cpu_possible_mask — assuming a shared interrupt must be routable to all CPUs. This creates a conflict on SMT platforms: the MSC has a restricted accessibility but a shared error interrupt, causing probe to fail with: msc:N is a private resource with a shared error interrupt This RFC patch removes the check to allow MSC probe to succeed on SMT platforms, but this is mainly intended to start a discussion. We would appreciate advice on the right way to handle this conflict. Is this a spec issue, or should the driver handle SPI error interrupts differently
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when an MSC has a restricted accessibility mask?
I expect the MPAM driver could be taught to deal with SPI interrupts without global affinity.
quoted hunk
Signed-off-by: Yadong Qi <redacted> --- drivers/resctrl/mpam_devices.c | 7 ------- 1 file changed, 7 deletions(-)diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c index b69f99488111..e8b2b5e00d7c 100644 --- a/drivers/resctrl/mpam_devices.c +++ b/drivers/resctrl/mpam_devices.c@@ -1964,13 +1964,6 @@ static int mpam_msc_setup_error_irq(struct mpam_msc *msc) if (irq_is_percpu(irq)) return __setup_ppi(msc); - /* sanity check: shared interrupts can be routed anywhere? */ - if (!cpumask_equal(&msc->accessibility, cpu_possible_mask)) { - pr_err_once("msc:%u is a private resource with a shared error interrupt", - msc->id); - return -EINVAL; - } -
To do this the MPAM driver would have to remove the interrupt from balancing, IRQF_NOBALANCING, and control the balancing itself. Thanks, Ben
return 0; }