Re: [PATCH kernel v9 28/32] powerpc/mmu: Add userspace-to-physical addresses translation cache

David Gibson
2015-04-29T22:33:26Z

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On Sat, Apr 25, 2015 at 10:14:52PM +1000, Alexey Kardashevskiy wrote:
> We are adding support for DMA memory pre-registration to be used in
> conjunction with VFIO. The idea is that the userspace which is going to
> run a guest may want to pre-register a user space memory region so
> it all gets pinned once and never goes away. Having this done,
> a hypervisor will not have to pin/unpin pages on every DMA map/unmap
> request. This is going to help with multiple pinning of the same memory
> and in-kernel acceleration of DMA requests.
>
> This adds a list of memory regions to mm_context_t. Each region consists
> of a header and a list of physical addresses. This adds API to:
> 1. register/unregister memory regions;
> 2. do final cleanup (which puts all pre-registered pages);
> 3. do userspace to physical address translation;
> 4. manage a mapped pages counter; when it is zero, it is safe to
> unregister the region.
>
> Multiple registration of the same region is allowed, kref is used to
> track the number of registrations.
>
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> ---
> Changes:
> v8:
> * s/mm_iommu_table_group_mem_t/struct mm_iommu_table_group_mem_t/
> * fixed error fallback look (s/[i]/[j]/)
> ---
> arch/powerpc/include/asm/mmu-hash64.h | 3 +
> arch/powerpc/include/asm/mmu_context.h | 17 +++
> arch/powerpc/mm/Makefile | 1 +
> arch/powerpc/mm/mmu_context_hash64.c | 6 +
> arch/powerpc/mm/mmu_context_hash64_iommu.c | 215 +++++++++++++++++++++++++++++
> 5 files changed, 242 insertions(+)
> create mode 100644 arch/powerpc/mm/mmu_context_hash64_iommu.c
>
> diff --git a/arch/powerpc/include/asm/mmu-hash64.h b/arch/powerpc/include/asm/mmu-hash64.h
> index 1da6a81..a82f534 100644
> --- a/arch/powerpc/include/asm/mmu-hash64.h
> +++ b/arch/powerpc/include/asm/mmu-hash64.h
> @@ -536,6 +536,9 @@ typedef struct {
> /* for 4K PTE fragment support */
> void *pte_frag;
> #endif
> +#ifdef CONFIG_SPAPR_TCE_IOMMU
> + struct list_head iommu_group_mem_list;
> +#endif

Urgh. I know I'm not one to talk, having done the hugepage crap in
there, but man mm_context_t has grown to a bloated mess from orginally
being just intended as a context ID integer :/.

> } mm_context_t;
>
>
> diff --git a/arch/powerpc/include/asm/mmu_context.h b/arch/powerpc/include/asm/mmu_context.h
> index 73382eb..d6116ca 100644
> --- a/arch/powerpc/include/asm/mmu_context.h
> +++ b/arch/powerpc/include/asm/mmu_context.h
> @@ -16,6 +16,23 @@
> */
> extern int init_new_context(struct task_struct *tsk, struct mm_struct *mm);
> extern void destroy_context(struct mm_struct *mm);
> +#ifdef CONFIG_SPAPR_TCE_IOMMU
> +struct mm_iommu_table_group_mem_t;
> +
> +extern bool mm_iommu_preregistered(void);
> +extern long mm_iommu_alloc(unsigned long ua, unsigned long entries,
> + struct mm_iommu_table_group_mem_t **pmem);
> +extern struct mm_iommu_table_group_mem_t *mm_iommu_get(unsigned long ua,
> + unsigned long entries);
> +extern long mm_iommu_put(struct mm_iommu_table_group_mem_t *mem);
> +extern void mm_iommu_cleanup(mm_context_t *ctx);
> +extern struct mm_iommu_table_group_mem_t *mm_iommu_lookup(unsigned long ua,
> + unsigned long size);
> +extern long mm_iommu_ua_to_hpa(struct mm_iommu_table_group_mem_t *mem,
> + unsigned long ua, unsigned long *hpa);
> +extern long mm_iommu_mapped_update(struct mm_iommu_table_group_mem_t *mem,
> + bool inc);
> +#endif
>
> extern void switch_mmu_context(struct mm_struct *prev, struct mm_struct *next);
> extern void switch_slb(struct task_struct *tsk, struct mm_struct *mm);
> diff --git a/arch/powerpc/mm/Makefile b/arch/powerpc/mm/Makefile
> index 9c8770b..e216704 100644
> --- a/arch/powerpc/mm/Makefile
> +++ b/arch/powerpc/mm/Makefile
> @@ -36,3 +36,4 @@ obj-$(CONFIG_PPC_SUBPAGE_PROT) += subpage-prot.o
> obj-$(CONFIG_NOT_COHERENT_CACHE) += dma-noncoherent.o
> obj-$(CONFIG_HIGHMEM) += highmem.o
> obj-$(CONFIG_PPC_COPRO_BASE) += copro_fault.o
> +obj-$(CONFIG_SPAPR_TCE_IOMMU) += mmu_context_hash64_iommu.o
> diff --git a/arch/powerpc/mm/mmu_context_hash64.c b/arch/powerpc/mm/mmu_context_hash64.c
> index 178876ae..eb3080c 100644
> --- a/arch/powerpc/mm/mmu_context_hash64.c
> +++ b/arch/powerpc/mm/mmu_context_hash64.c
> @@ -89,6 +89,9 @@ int init_new_context(struct task_struct *tsk, struct mm_struct *mm)
> #ifdef CONFIG_PPC_64K_PAGES
> mm->context.pte_frag = NULL;
> #endif
> +#ifdef CONFIG_SPAPR_TCE_IOMMU
> + INIT_LIST_HEAD_RCU(&mm->context.iommu_group_mem_list);
> +#endif
> return 0;
> }
>
> @@ -132,6 +135,9 @@ static inline void destroy_pagetable_page(struct mm_struct *mm)
>
> void destroy_context(struct mm_struct *mm)
> {
> +#ifdef CONFIG_SPAPR_TCE_IOMMU
> + mm_iommu_cleanup(&mm->context);
> +#endif
>
> #ifdef CONFIG_PPC_ICSWX
> drop_cop(mm->context.acop, mm);
> diff --git a/arch/powerpc/mm/mmu_context_hash64_iommu.c b/arch/powerpc/mm/mmu_context_hash64_iommu.c
> new file mode 100644
> index 0000000..af7668c
> --- /dev/null
> +++ b/arch/powerpc/mm/mmu_context_hash64_iommu.c
> @@ -0,0 +1,215 @@
> +/*
> + * IOMMU helpers in MMU context.
> + *
> + * Copyright (C) 2015 IBM Corp. <aik@ozlabs.ru>
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU General Public License
> + * as published by the Free Software Foundation; either version
> + * 2 of the License, or (at your option) any later version.
> + *
> + */
> +
> +#include <linux/sched.h>
> +#include <linux/slab.h>
> +#include <linux/rculist.h>
> +#include <linux/vmalloc.h>
> +#include <linux/kref.h>
> +#include <asm/mmu_context.h>
> +
> +struct mm_iommu_table_group_mem_t {
> + struct list_head next;
> + struct rcu_head rcu;
> + struct kref kref; /* one reference per VFIO container */
> + atomic_t mapped; /* number of currently mapped pages */
> + u64 ua; /* userspace address */
> + u64 entries; /* number of entries in hpas[] */

Maybe 'npages', since this is used to determine the range of user
addresses covered, not just the number of entries in hpas.

> + u64 *hpas; /* vmalloc'ed */
> +};
> +
> +bool mm_iommu_preregistered(void)
> +{
> + if (!current || !current->mm)
> + return false;
> +
> + return !list_empty(&current->mm->context.iommu_group_mem_list);
> +}
> +EXPORT_SYMBOL_GPL(mm_iommu_preregistered);
> +
> +long mm_iommu_alloc(unsigned long ua, unsigned long entries,
> + struct mm_iommu_table_group_mem_t **pmem)
> +{
> + struct mm_iommu_table_group_mem_t *mem;
> + long i, j;
> + struct page *page = NULL;
> +
> + list_for_each_entry_rcu(mem, &current->mm->context.iommu_group_mem_list,
> + next) {
> + if ((mem->ua == ua) && (mem->entries == entries))
> + return -EBUSY;
> +
> + /* Overlap? */
> + if ((mem->ua < (ua + (entries << PAGE_SHIFT))) &&
> + (ua < (mem->ua + (mem->entries << PAGE_SHIFT))))
> + return -EINVAL;
> + }
> +
> + mem = kzalloc(sizeof(*mem), GFP_KERNEL);
> + if (!mem)
> + return -ENOMEM;
> +
> + mem->hpas = vzalloc(entries * sizeof(mem->hpas[0]));
> + if (!mem->hpas) {
> + kfree(mem);
> + return -ENOMEM;
> + }
> +
> + for (i = 0; i < entries; ++i) {
> + if (1 != get_user_pages_fast(ua + (i << PAGE_SHIFT),
> + 1/* pages */, 1/* iswrite */, &page)) {

Do you really need to call gup() in a loop? It can do more than one
page at a time..

That might work better if you kept a list of struct page *s instead of
hpas.

> + for (j = 0; j < i; ++j)
> + put_page(pfn_to_page(
> + mem->hpas[j] >> PAGE_SHIFT));
> + vfree(mem->hpas);
> + kfree(mem);
> + return -EFAULT;
> + }
> +
> + mem->hpas[i] = page_to_pfn(page) << PAGE_SHIFT;
> + }
> +
> + kref_init(&mem->kref);
> + atomic_set(&mem->mapped, 0);
> + mem->ua = ua;
> + mem->entries = entries;
> + *pmem = mem;
> +
> + list_add_rcu(&mem->next, &current->mm->context.iommu_group_mem_list);
> +
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(mm_iommu_alloc);
> +
> +static void mm_iommu_unpin(struct mm_iommu_table_group_mem_t *mem)
> +{
> + long i;
> + struct page *page = NULL;
> +
> + for (i = 0; i < mem->entries; ++i) {
> + if (!mem->hpas[i])
> + continue;
> +
> + page = pfn_to_page(mem->hpas[i] >> PAGE_SHIFT);
> + if (!page)
> + continue;
> +
> + put_page(page);
> + mem->hpas[i] = 0;
> + }
> +}
> +
> +static void mm_iommu_free(struct rcu_head *head)
> +{
> + struct mm_iommu_table_group_mem_t *mem = container_of(head,
> + struct mm_iommu_table_group_mem_t, rcu);
> +
> + mm_iommu_unpin(mem);
> + vfree(mem->hpas);
> + kfree(mem);
> +}
> +
> +static void mm_iommu_release(struct kref *kref)
> +{
> + struct mm_iommu_table_group_mem_t *mem = container_of(kref,
> + struct mm_iommu_table_group_mem_t, kref);
> +
> + list_del_rcu(&mem->next);
> + call_rcu(&mem->rcu, mm_iommu_free);
> +}
> +
> +struct mm_iommu_table_group_mem_t *mm_iommu_get(unsigned long ua,
> + unsigned long entries)
> +{
> + struct mm_iommu_table_group_mem_t *mem;
> +
> + list_for_each_entry_rcu(mem, &current->mm->context.iommu_group_mem_list,
> + next) {
> + if ((mem->ua == ua) && (mem->entries == entries)) {
> + kref_get(&mem->kref);
> + return mem;
> + }
> + }
> +
> + return NULL;
> +}
> +EXPORT_SYMBOL_GPL(mm_iommu_get);
> +
> +long mm_iommu_put(struct mm_iommu_table_group_mem_t *mem)
> +{
> + if (atomic_read(&mem->mapped))
> + return -EBUSY;

What prevents a race between the atomic_read() above and the release below?

> + kref_put(&mem->kref, mm_iommu_release);
> +
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(mm_iommu_put);
> +
> +struct mm_iommu_table_group_mem_t *mm_iommu_lookup(unsigned long ua,
> + unsigned long size)
> +{
> + struct mm_iommu_table_group_mem_t *mem, *ret = NULL;
> +
> + list_for_each_entry_rcu(mem,
> + &current->mm->context.iommu_group_mem_list,
> + next) {
> + if ((mem->ua <= ua) &&
> + (ua + size <= mem->ua +
> + (mem->entries << PAGE_SHIFT))) {
> + ret = mem;
> + break;
> + }
> + }
> +
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(mm_iommu_lookup);
> +
> +long mm_iommu_ua_to_hpa(struct mm_iommu_table_group_mem_t *mem,
> + unsigned long ua, unsigned long *hpa)

Return type should be int, it's just an error code.

> +{
> + const long entry = (ua - mem->ua) >> PAGE_SHIFT;
> + u64 *va = &mem->hpas[entry];
> +
> + if (entry >= mem->entries)
> + return -EFAULT;
> +
> + *hpa = *va | (ua & ~PAGE_MASK);
> +
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(mm_iommu_ua_to_hpa);
> +
> +long mm_iommu_mapped_update(struct mm_iommu_table_group_mem_t *mem, bool inc)
> +{
> + long ret = 0;
> +
> + if (inc)
> + atomic_inc(&mem->mapped);
> + else
> + ret = atomic_dec_if_positive(&mem->mapped);
> +
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(mm_iommu_mapped_update);

I think this would be clearer as separate inc and dec functions.

> +
> +void mm_iommu_cleanup(mm_context_t *ctx)
> +{
> + while (!list_empty(&ctx->iommu_group_mem_list)) {
> + struct mm_iommu_table_group_mem_t *mem;
> +
> + mem = list_first_entry(&ctx->iommu_group_mem_list,
> + struct mm_iommu_table_group_mem_t, next);
> + mm_iommu_release(&mem->kref);
> + }
> +}

--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson

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