On Mon, 24 Feb 2020 19:23:42 +0100 Jean-Philippe Brucker <[email protected]> wrote:
> From: Jean-Philippe Brucker <[email protected]> > > To enable address space sharing with the IOMMU, introduce mm_context_get() > and 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 <[email protected]> A few more trivial points. Thanks, Jonathan > --- > v2->v4: handle KPTI > --- > arch/arm64/include/asm/mmu.h | 1 + > arch/arm64/include/asm/mmu_context.h | 11 ++- > arch/arm64/mm/context.c | 103 +++++++++++++++++++++++++-- > 3 files changed, 109 insertions(+), 6 deletions(-) > > diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h > index e4d862420bb4..70ac3d4cbd3e 100644 > --- a/arch/arm64/include/asm/mmu.h > +++ b/arch/arm64/include/asm/mmu.h > @@ -18,6 +18,7 @@ > > typedef struct { > atomic64_t id; > + unsigned long pinned; > void *vdso; > unsigned long flags; > } mm_context_t; > diff --git a/arch/arm64/include/asm/mmu_context.h > b/arch/arm64/include/asm/mmu_context.h > index 3827ff4040a3..70715c10c02a 100644 > --- a/arch/arm64/include/asm/mmu_context.h > +++ b/arch/arm64/include/asm/mmu_context.h > @@ -175,7 +175,13 @@ static inline void cpu_replace_ttbr1(pgd_t *pgdp) > #define destroy_context(mm) do { } while(0) > void check_and_switch_context(struct mm_struct *mm, unsigned int cpu); > > -#define init_new_context(tsk,mm) ({ atomic64_set(&(mm)->context.id, 0); > 0; }) > +static inline int > +init_new_context(struct task_struct *tsk, struct mm_struct *mm) > +{ > + atomic64_set(&mm->context.id, 0); > + mm->context.pinned = 0; > + return 0; > +} > > #ifdef CONFIG_ARM64_SW_TTBR0_PAN > static inline void update_saved_ttbr0(struct task_struct *tsk, > @@ -248,6 +254,9 @@ switch_mm(struct mm_struct *prev, struct mm_struct *next, > void verify_cpu_asid_bits(void); > void post_ttbr_update_workaround(void); > > +unsigned long mm_context_get(struct mm_struct *mm); > +void mm_context_put(struct mm_struct *mm); > + > #endif /* !__ASSEMBLY__ */ > > #endif /* !__ASM_MMU_CONTEXT_H */ > diff --git a/arch/arm64/mm/context.c b/arch/arm64/mm/context.c > index 121aba5b1941..5558de88b67d 100644 > --- a/arch/arm64/mm/context.c > +++ b/arch/arm64/mm/context.c > @@ -26,6 +26,10 @@ static DEFINE_PER_CPU(atomic64_t, active_asids); > static DEFINE_PER_CPU(u64, reserved_asids); > static cpumask_t tlb_flush_pending; > > +static unsigned long max_pinned_asids; > +static unsigned long nr_pinned_asids; > +static unsigned long *pinned_asid_map; > + > #define ASID_MASK (~GENMASK(asid_bits - 1, 0)) > #define ASID_FIRST_VERSION (1UL << asid_bits) > > @@ -73,6 +77,9 @@ void verify_cpu_asid_bits(void) > > static void set_kpti_asid_bits(void) > { > + unsigned int k; > + u8 *dst = (u8 *)asid_map; > + u8 *src = (u8 *)pinned_asid_map; > unsigned int len = BITS_TO_LONGS(NUM_USER_ASIDS) * sizeof(unsigned > long); > /* > * In case of KPTI kernel/user ASIDs are allocated in > @@ -80,7 +87,8 @@ static void set_kpti_asid_bits(void) > * is set, then the ASID will map only userspace. Thus > * mark even as reserved for kernel. > */ > - memset(asid_map, 0xaa, len); > + for (k = 0; k < len; k++) > + dst[k] = src[k] | 0xaa; > } > > static void set_reserved_asid_bits(void) > @@ -88,9 +96,12 @@ static void set_reserved_asid_bits(void) > if (arm64_kernel_unmapped_at_el0()) > set_kpti_asid_bits(); > else > - bitmap_clear(asid_map, 0, NUM_USER_ASIDS); > + bitmap_copy(asid_map, pinned_asid_map, NUM_USER_ASIDS); > } > > +#define asid_gen_match(asid) \ > + (!(((asid) ^ atomic64_read(&asid_generation)) >> asid_bits)) > + I'd have slightly preferred this bit of refactoring as a precursor patch. > static void flush_context(void) > { > int i; > @@ -161,6 +172,14 @@ static u64 new_context(struct mm_struct *mm) > if (check_update_reserved_asid(asid, newasid)) > return newasid; > > + /* > + * If it is pinned, we can keep using it. Note that reserved > + * takes priority, because even if it is also pinned, we need to > + * update the generation into the reserved_asids. > + */ > + if (mm->context.pinned) > + return newasid; > + > /* > * We had a valid ASID in a previous life, so try to re-use > * it if possible. > @@ -219,8 +238,7 @@ void check_and_switch_context(struct mm_struct *mm, > unsigned int cpu) > * because atomic RmWs are totally ordered for a given location. > */ > old_active_asid = atomic64_read(&per_cpu(active_asids, cpu)); > - if (old_active_asid && > - !((asid ^ atomic64_read(&asid_generation)) >> asid_bits) && > + if (old_active_asid && asid_gen_match(asid) && > atomic64_cmpxchg_relaxed(&per_cpu(active_asids, cpu), > old_active_asid, asid)) > goto switch_mm_fastpath; > @@ -228,7 +246,7 @@ void check_and_switch_context(struct mm_struct *mm, > unsigned int cpu) > 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) { > + if (!asid_gen_match(asid)) { > asid = new_context(mm); > atomic64_set(&mm->context.id, asid); > } > @@ -251,6 +269,68 @@ void check_and_switch_context(struct mm_struct *mm, > unsigned int cpu) > cpu_switch_mm(mm->pgd, mm); > } > > +unsigned long mm_context_get(struct mm_struct *mm) > +{ > + unsigned long flags; > + u64 asid; > + > + raw_spin_lock_irqsave(&cpu_asid_lock, flags); > + > + asid = atomic64_read(&mm->context.id); > + > + if (mm->context.pinned) { > + mm->context.pinned++; > + asid &= ~ASID_MASK; > + goto out_unlock; > + } > + > + if (nr_pinned_asids >= max_pinned_asids) { > + asid = 0; > + goto out_unlock; > + } > + > + if (!asid_gen_match(asid)) { > + /* > + * We went through one or more rollover since that ASID was > + * used. Ensure that it is still valid, or generate a new one. > + */ > + asid = new_context(mm); > + atomic64_set(&mm->context.id, asid); > + } > + > + asid &= ~ASID_MASK; > + > + nr_pinned_asids++; > + __set_bit(asid2idx(asid), pinned_asid_map); > + mm->context.pinned++; > + > +out_unlock: > + raw_spin_unlock_irqrestore(&cpu_asid_lock, flags); > + > + /* Set the equivalent of USER_ASID_BIT */ > + if (asid && IS_ENABLED(CONFIG_UNMAP_KERNEL_AT_EL0)) > + asid |= 1; > + > + return asid; > +} > +EXPORT_SYMBOL_GPL(mm_context_get); > + > +void mm_context_put(struct mm_struct *mm) > +{ > + unsigned long flags; > + u64 asid = atomic64_read(&mm->context.id) & ~ASID_MASK; > + > + raw_spin_lock_irqsave(&cpu_asid_lock, flags); > + > + if (--mm->context.pinned == 0) { > + __clear_bit(asid2idx(asid), pinned_asid_map); > + nr_pinned_asids--; > + } > + > + raw_spin_unlock_irqrestore(&cpu_asid_lock, flags); > +} > +EXPORT_SYMBOL_GPL(mm_context_put); > + > /* Errata workaround post TTBRx_EL1 update. */ > asmlinkage void post_ttbr_update_workaround(void) > { > @@ -279,6 +359,19 @@ static int asids_init(void) > panic("Failed to allocate bitmap for %lu ASIDs\n", > NUM_USER_ASIDS); > > + pinned_asid_map = kcalloc(BITS_TO_LONGS(NUM_USER_ASIDS), > + sizeof(*pinned_asid_map), GFP_KERNEL); > + if (!pinned_asid_map) > + panic("Failed to allocate pinned bitmap\n"); Perhaps "Failed to allocate pinnned asid bitmap\n" > + > + /* > + * We assume that an ASID is always available after a rollover. This > + * means that even if all CPUs have a reserved ASID, there still is at > + * least one slot available in the asid map. > + */ > + max_pinned_asids = num_available_asids - num_possible_cpus() - 1; > + nr_pinned_asids = 0; > + > /* > * We cannot call set_reserved_asid_bits() here because CPU > * caps are not finalized yet, so it is safer to assume KPTI _______________________________________________ iommu mailing list [email protected] https://lists.linuxfoundation.org/mailman/listinfo/iommu
