This is a note to let you know that I've just added the patch titled powerpc/xics: Work around limitations of OPAL XICS priority handling to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: powerpc-xics-work-around-limitations-of-opal-xics-priority-handling.patch and it can be found in the queue-4.9 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From a69e2fb70350a66f91175cd2625f1e8215c5b6e9 Mon Sep 17 00:00:00 2001 From: Balbir Singh <bsingharora@xxxxxxxxx> Date: Fri, 3 Mar 2017 11:58:44 +1100 Subject: powerpc/xics: Work around limitations of OPAL XICS priority handling From: Balbir Singh <bsingharora@xxxxxxxxx> commit a69e2fb70350a66f91175cd2625f1e8215c5b6e9 upstream. The CPPR (Current Processor Priority Register) of a XICS interrupt presentation controller contains a value N, such that only interrupts with a priority "more favoured" than N will be received by the CPU, where "more favoured" means "less than". So if the CPPR has the value 5 then only interrupts with a priority of 0-4 inclusive will be received. In theory the CPPR can support a value of 0 to 255 inclusive. In practice Linux only uses values of 0, 4, 5 and 0xff. Setting the CPPR to 0 rejects all interrupts, setting it to 0xff allows all interrupts. The values 4 and 5 are used to differentiate IPIs from external interrupts. Setting the CPPR to 5 allows IPIs to be received but not external interrupts. The CPPR emulation in the OPAL XICS implementation only directly supports priorities 0 and 0xff. All other priorities are considered equivalent, and mapped to a single priority value internally. This means when using icp-opal we can not allow IPIs but not externals. This breaks Linux's use of priority values when a CPU is hot unplugged. After migrating IRQs away from the CPU that is being offlined, we set the priority to 5, meaning we still want the offline CPU to receive IPIs. But the effect of the OPAL XICS emulation's use of a single priority value is that all interrupts are rejected by the CPU. With the CPU offline, and not receiving IPIs, we may not be able to wake it up to bring it back online. The first part of the fix is in icp_opal_set_cpu_priority(). CPPR values of 0 to 4 inclusive will correctly cause all interrupts to be rejected, so we pass those CPPR values through to OPAL. However if we are called with a CPPR of 5 or greater, the caller is expecting to be able to allow IPIs but not external interrupts. We know this doesn't work, so instead of rejecting all interrupts we choose the opposite which is to allow all interrupts. This is still not correct behaviour, but we know for the only existing caller (xics_migrate_irqs_away()), that it is the better option. The other part of the fix is in xics_migrate_irqs_away(). Instead of setting priority (CPPR) to 0, and then back to 5 before migrating IRQs, we migrate the IRQs before setting the priority back to 5. This should have no effect on an ICP backend with a working set_priority(), and on icp-opal it means we will keep all interrupts blocked until after we've finished doing the IRQ migration. Additionally we wait for 5ms after doing the migration to make sure there are no IRQs in flight. Fixes: d74361881f0d ("powerpc/xics: Add ICP OPAL backend") Suggested-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx> Reported-by: Vaidyanathan Srinivasan <svaidy@xxxxxxxxxxxxxxxxxx> Tested-by: Vaidyanathan Srinivasan <svaidy@xxxxxxxxxxxxxxxxxx> Signed-off-by: Balbir Singh <bsingharora@xxxxxxxxx> [mpe: Rewrote comments and change log, change delay to 5ms] Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- arch/powerpc/sysdev/xics/icp-opal.c | 10 ++++++++++ arch/powerpc/sysdev/xics/xics-common.c | 17 ++++++++++++++--- 2 files changed, 24 insertions(+), 3 deletions(-) --- a/arch/powerpc/sysdev/xics/icp-opal.c +++ b/arch/powerpc/sysdev/xics/icp-opal.c @@ -91,6 +91,16 @@ static unsigned int icp_opal_get_irq(voi static void icp_opal_set_cpu_priority(unsigned char cppr) { + /* + * Here be dragons. The caller has asked to allow only IPI's and not + * external interrupts. But OPAL XIVE doesn't support that. So instead + * of allowing no interrupts allow all. That's still not right, but + * currently the only caller who does this is xics_migrate_irqs_away() + * and it works in that case. + */ + if (cppr >= DEFAULT_PRIORITY) + cppr = LOWEST_PRIORITY; + xics_set_base_cppr(cppr); opal_int_set_cppr(cppr); iosync(); --- a/arch/powerpc/sysdev/xics/xics-common.c +++ b/arch/powerpc/sysdev/xics/xics-common.c @@ -20,6 +20,7 @@ #include <linux/of.h> #include <linux/slab.h> #include <linux/spinlock.h> +#include <linux/delay.h> #include <asm/prom.h> #include <asm/io.h> @@ -198,9 +199,6 @@ void xics_migrate_irqs_away(void) /* Remove ourselves from the global interrupt queue */ xics_set_cpu_giq(xics_default_distrib_server, 0); - /* Allow IPIs again... */ - icp_ops->set_priority(DEFAULT_PRIORITY); - for_each_irq_desc(virq, desc) { struct irq_chip *chip; long server; @@ -255,6 +253,19 @@ void xics_migrate_irqs_away(void) unlock: raw_spin_unlock_irqrestore(&desc->lock, flags); } + + /* Allow "sufficient" time to drop any inflight IRQ's */ + mdelay(5); + + /* + * Allow IPIs again. This is done at the very end, after migrating all + * interrupts, the expectation is that we'll only get woken up by an IPI + * interrupt beyond this point, but leave externals masked just to be + * safe. If we're using icp-opal this may actually allow all + * interrupts anyway, but that should be OK. + */ + icp_ops->set_priority(DEFAULT_PRIORITY); + } #endif /* CONFIG_HOTPLUG_CPU */ Patches currently in stable-queue which might be from bsingharora@xxxxxxxxx are queue-4.9/powerpc-xics-work-around-limitations-of-opal-xics-priority-handling.patch