From 5aec715d7d3122f77cabaa7578d9d25a0c1ed20e Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Tue, 6 Oct 2015 18:46:24 +0100 Subject: arm64: mm: rewrite ASID allocator and MM context-switching code Our current switch_mm implementation suffers from a number of problems: (1) The ASID allocator relies on IPIs to synchronise the CPUs on a rollover event (2) Because of (1), we cannot allocate ASIDs with interrupts disabled and therefore make use of a TIF_SWITCH_MM flag to postpone the actual switch to finish_arch_post_lock_switch (3) We run context switch with a reserved (invalid) TTBR0 value, even though the ASID and pgd are updated atomically (4) We take a global spinlock (cpu_asid_lock) during context-switch (5) We use h/w broadcast TLB operations when they are not required (e.g. in flush_context) This patch addresses these problems by rewriting the ASID algorithm to match the bitmap-based arch/arm/ implementation more closely. This in turn allows us to remove much of the complications surrounding switch_mm, including the ugly thread flag. Reviewed-by: Catalin Marinas Signed-off-by: Will Deacon Signed-off-by: Catalin Marinas --- arch/arm64/mm/context.c | 238 +++++++++++++++++++++++++++++------------------- 1 file changed, 145 insertions(+), 93 deletions(-) (limited to 'arch/arm64/mm/context.c') diff --git a/arch/arm64/mm/context.c b/arch/arm64/mm/context.c index 48b53fb381af..e902229b1a3d 100644 --- a/arch/arm64/mm/context.c +++ b/arch/arm64/mm/context.c @@ -17,135 +17,187 @@ * along with this program. If not, see . */ -#include +#include #include +#include #include -#include -#include +#include #include #include -#include -#define asid_bits(reg) \ - (((read_cpuid(ID_AA64MMFR0_EL1) & 0xf0) >> 2) + 8) +static u32 asid_bits; +static DEFINE_RAW_SPINLOCK(cpu_asid_lock); -#define ASID_FIRST_VERSION (1 << MAX_ASID_BITS) +static atomic64_t asid_generation; +static unsigned long *asid_map; -static DEFINE_RAW_SPINLOCK(cpu_asid_lock); -unsigned int cpu_last_asid = ASID_FIRST_VERSION; +static DEFINE_PER_CPU(atomic64_t, active_asids); +static DEFINE_PER_CPU(u64, reserved_asids); +static cpumask_t tlb_flush_pending; -/* - * We fork()ed a process, and we need a new context for the child to run in. - */ -void __init_new_context(struct task_struct *tsk, struct mm_struct *mm) +#define ASID_MASK (~GENMASK(asid_bits - 1, 0)) +#define ASID_FIRST_VERSION (1UL << asid_bits) +#define NUM_USER_ASIDS ASID_FIRST_VERSION + +static void flush_context(unsigned int cpu) { - mm->context.id = 0; - raw_spin_lock_init(&mm->context.id_lock); + int i; + u64 asid; + + /* Update the list of reserved ASIDs and the ASID bitmap. */ + bitmap_clear(asid_map, 0, NUM_USER_ASIDS); + + /* + * Ensure the generation bump is observed before we xchg the + * active_asids. + */ + smp_wmb(); + + for_each_possible_cpu(i) { + asid = atomic64_xchg_relaxed(&per_cpu(active_asids, i), 0); + /* + * If this CPU has already been through a + * rollover, but hasn't run another task in + * the meantime, we must preserve its reserved + * ASID, as this is the only trace we have of + * the process it is still running. + */ + if (asid == 0) + asid = per_cpu(reserved_asids, i); + __set_bit(asid & ~ASID_MASK, asid_map); + per_cpu(reserved_asids, i) = asid; + } + + /* Queue a TLB invalidate and flush the I-cache if necessary. */ + cpumask_setall(&tlb_flush_pending); + + if (icache_is_aivivt()) + __flush_icache_all(); } -static void flush_context(void) +static int is_reserved_asid(u64 asid) { - /* set the reserved TTBR0 before flushing the TLB */ - cpu_set_reserved_ttbr0(); - local_flush_tlb_all(); - if (icache_is_aivivt()) - __local_flush_icache_all(); + int cpu; + for_each_possible_cpu(cpu) + if (per_cpu(reserved_asids, cpu) == asid) + return 1; + return 0; } -static void set_mm_context(struct mm_struct *mm, unsigned int asid) +static u64 new_context(struct mm_struct *mm, unsigned int cpu) { - unsigned long flags; + static u32 cur_idx = 1; + u64 asid = atomic64_read(&mm->context.id); + u64 generation = atomic64_read(&asid_generation); - /* - * Locking needed for multi-threaded applications where the same - * mm->context.id could be set from different CPUs during the - * broadcast. This function is also called via IPI so the - * mm->context.id_lock has to be IRQ-safe. - */ - raw_spin_lock_irqsave(&mm->context.id_lock, flags); - if (likely((mm->context.id ^ cpu_last_asid) >> MAX_ASID_BITS)) { + if (asid != 0) { /* - * Old version of ASID found. Set the new one and reset - * mm_cpumask(mm). + * If our current ASID was active during a rollover, we + * can continue to use it and this was just a false alarm. */ - mm->context.id = asid; - cpumask_clear(mm_cpumask(mm)); + if (is_reserved_asid(asid)) + return generation | (asid & ~ASID_MASK); + + /* + * We had a valid ASID in a previous life, so try to re-use + * it if possible. + */ + asid &= ~ASID_MASK; + if (!__test_and_set_bit(asid, asid_map)) + goto bump_gen; } - raw_spin_unlock_irqrestore(&mm->context.id_lock, flags); /* - * Set the mm_cpumask(mm) bit for the current CPU. + * Allocate a free ASID. If we can't find one, take a note of the + * currently active ASIDs and mark the TLBs as requiring flushes. + * We always count from ASID #1, as we use ASID #0 when setting a + * reserved TTBR0 for the init_mm. */ - cpumask_set_cpu(smp_processor_id(), mm_cpumask(mm)); + asid = find_next_zero_bit(asid_map, NUM_USER_ASIDS, cur_idx); + if (asid != NUM_USER_ASIDS) + goto set_asid; + + /* We're out of ASIDs, so increment the global generation count */ + generation = atomic64_add_return_relaxed(ASID_FIRST_VERSION, + &asid_generation); + flush_context(cpu); + + /* We have at least 1 ASID per CPU, so this will always succeed */ + asid = find_next_zero_bit(asid_map, NUM_USER_ASIDS, 1); + +set_asid: + __set_bit(asid, asid_map); + cur_idx = asid; + +bump_gen: + asid |= generation; + cpumask_clear(mm_cpumask(mm)); + return asid; } -/* - * Reset the ASID on the current CPU. This function call is broadcast from the - * CPU handling the ASID rollover and holding cpu_asid_lock. - */ -static void reset_context(void *info) +void check_and_switch_context(struct mm_struct *mm, unsigned int cpu) { - unsigned int asid; - unsigned int cpu = smp_processor_id(); - struct mm_struct *mm = current->active_mm; + unsigned long flags; + u64 asid; + + asid = atomic64_read(&mm->context.id); /* - * current->active_mm could be init_mm for the idle thread immediately - * after secondary CPU boot or hotplug. TTBR0_EL1 is already set to - * the reserved value, so no need to reset any context. + * The memory ordering here is subtle. We rely on the control + * dependency between the generation read and the update of + * active_asids to ensure that we are synchronised with a + * parallel rollover (i.e. this pairs with the smp_wmb() in + * flush_context). */ - if (mm == &init_mm) - return; + if (!((asid ^ atomic64_read(&asid_generation)) >> asid_bits) + && atomic64_xchg_relaxed(&per_cpu(active_asids, cpu), asid)) + goto switch_mm_fastpath; + + raw_spin_lock_irqsave(&cpu_asid_lock, flags); + /* Check that our ASID belongs to the current generation. */ + asid = atomic64_read(&mm->context.id); + if ((asid ^ atomic64_read(&asid_generation)) >> asid_bits) { + asid = new_context(mm, cpu); + atomic64_set(&mm->context.id, asid); + } - smp_rmb(); - asid = cpu_last_asid + cpu; + if (cpumask_test_and_clear_cpu(cpu, &tlb_flush_pending)) + local_flush_tlb_all(); - flush_context(); - set_mm_context(mm, asid); + atomic64_set(&per_cpu(active_asids, cpu), asid); + cpumask_set_cpu(cpu, mm_cpumask(mm)); + raw_spin_unlock_irqrestore(&cpu_asid_lock, flags); - /* set the new ASID */ +switch_mm_fastpath: cpu_switch_mm(mm->pgd, mm); } -void __new_context(struct mm_struct *mm) +static int asids_init(void) { - unsigned int asid; - unsigned int bits = asid_bits(); - - raw_spin_lock(&cpu_asid_lock); - /* - * Check the ASID again, in case the change was broadcast from another - * CPU before we acquired the lock. - */ - if (!unlikely((mm->context.id ^ cpu_last_asid) >> MAX_ASID_BITS)) { - cpumask_set_cpu(smp_processor_id(), mm_cpumask(mm)); - raw_spin_unlock(&cpu_asid_lock); - return; - } - /* - * At this point, it is guaranteed that the current mm (with an old - * ASID) isn't active on any other CPU since the ASIDs are changed - * simultaneously via IPI. - */ - asid = ++cpu_last_asid; - - /* - * If we've used up all our ASIDs, we need to start a new version and - * flush the TLB. - */ - if (unlikely((asid & ((1 << bits) - 1)) == 0)) { - /* increment the ASID version */ - cpu_last_asid += (1 << MAX_ASID_BITS) - (1 << bits); - if (cpu_last_asid == 0) - cpu_last_asid = ASID_FIRST_VERSION; - asid = cpu_last_asid + smp_processor_id(); - flush_context(); - smp_wmb(); - smp_call_function(reset_context, NULL, 1); - cpu_last_asid += NR_CPUS - 1; + int fld = cpuid_feature_extract_field(read_cpuid(ID_AA64MMFR0_EL1), 4); + + switch (fld) { + default: + pr_warn("Unknown ASID size (%d); assuming 8-bit\n", fld); + /* Fallthrough */ + case 0: + asid_bits = 8; + break; + case 2: + asid_bits = 16; } - set_mm_context(mm, asid); - raw_spin_unlock(&cpu_asid_lock); + /* If we end up with more CPUs than ASIDs, expect things to crash */ + WARN_ON(NUM_USER_ASIDS < num_possible_cpus()); + atomic64_set(&asid_generation, ASID_FIRST_VERSION); + asid_map = kzalloc(BITS_TO_LONGS(NUM_USER_ASIDS) * sizeof(*asid_map), + GFP_KERNEL); + if (!asid_map) + panic("Failed to allocate bitmap for %lu ASIDs\n", + NUM_USER_ASIDS); + + pr_info("ASID allocator initialised with %lu entries\n", NUM_USER_ASIDS); + return 0; } +early_initcall(asids_init); -- cgit v1.2.3