From 48118151d8cc7a457f61d91df0c053473d4b31b1 Mon Sep 17 00:00:00 2001 From: Jean-Philippe Brucker Date: Fri, 18 Sep 2020 12:18:44 +0200 Subject: arm64: mm: Pin down ASIDs for sharing mm with devices To enable address space sharing with the IOMMU, introduce arm64_mm_context_get() and arm64_mm_context_put(), that pin down a context and ensure that it will keep its ASID after a rollover. Export the symbols to let the modular SMMUv3 driver use them. Pinning is necessary because a device constantly needs a valid ASID, unlike tasks that only require one when running. Without pinning, we would need to notify the IOMMU when we're about to use a new ASID for a task, and it would get complicated when a new task is assigned a shared ASID. Consider the following scenario with no ASID pinned: 1. Task t1 is running on CPUx with shared ASID (gen=1, asid=1) 2. Task t2 is scheduled on CPUx, gets ASID (1, 2) 3. Task tn is scheduled on CPUy, a rollover occurs, tn gets ASID (2, 1) We would now have to immediately generate a new ASID for t1, notify the IOMMU, and finally enable task tn. We are holding the lock during all that time, since we can't afford having another CPU trigger a rollover. The IOMMU issues invalidation commands that can take tens of milliseconds. It gets needlessly complicated. All we wanted to do was schedule task tn, that has no business with the IOMMU. By letting the IOMMU pin tasks when needed, we avoid stalling the slow path, and let the pinning fail when we're out of shareable ASIDs. After a rollover, the allocator expects at least one ASID to be available in addition to the reserved ones (one per CPU). So (NR_ASIDS - NR_CPUS - 1) is the maximum number of ASIDs that can be shared with the IOMMU. Signed-off-by: Jean-Philippe Brucker Reviewed-by: Jonathan Cameron Link: https://lore.kernel.org/r/20200918101852.582559-5-jean-philippe@linaro.org Signed-off-by: Will Deacon --- arch/arm64/include/asm/mmu.h | 3 +++ 1 file changed, 3 insertions(+) (limited to 'arch/arm64/include/asm/mmu.h') diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h index a7a5ecaa2e83..0fda85b2cc1b 100644 --- a/arch/arm64/include/asm/mmu.h +++ b/arch/arm64/include/asm/mmu.h @@ -17,11 +17,14 @@ #ifndef __ASSEMBLY__ +#include + typedef struct { atomic64_t id; #ifdef CONFIG_COMPAT void *sigpage; #endif + refcount_t pinned; void *vdso; unsigned long flags; } mm_context_t; -- cgit v1.2.3