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

From: Alexey Kardashevskiy <[email-masked]>
Date: Fri, 01 May 2015 21:26:48 +1000

On 04/29/2015 05:01 PM, David Gibson wrote:
> 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 <[email-masked]>
>> ---
>> 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 :/.

Where else to put it then?... The other way to go would be some global map 
of pid<->iommu_group_mem_list which needs to be available from both VFIO 
and KVM.

>>   } 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. <[email-masked]>
>> + *
>> + *  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.

Hm. Ok :)

>> +	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..

Ufff. gup() returns the number of pages pinned or -errno if none. So if the 
return value is positive but less than the requested number of pages, it is 
still an error. Functions like this make me nervous :(

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

I only need struct page* when release the registered area. In other cases I 
just need fast conversion from an userspace address to a host physical 
address, including real mode. Ideally I would have to use page_address() 
which will work in real mode in my case but in general it does not have to. 
Using addresses rather than page structs makes it more explicit - I need an 
address, I store an address, simple.

I can change to page structs if you think it makes more sense, should I?

>> +			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?

Ouch. Nothing. And I cannot think of any nice fast solution here...
I can remove @mapped at all and do kref_get/put(&mem->kref) instead; a 
container will hold one reference too. And add a flag to 
mm_iommu_table_group_mem_t to know if mm_iommu_release has been called - 
this way I will know that was the very last reference, otherwise I'll 
return -EBUSY.

Or change mm_iommu_lookup() to do kref_get() and require every caller of it 
also call mm_iommu_put() and only call mm_iommu_mapped_update() when the 
reference is elevated. And change mm_iommu_put() to return a special code 
if that was the very last put() (will be checked by 
VFIO_IOMMU_SPAPR_UNREGISTER_MEMORY handler only, others would not care).

Any ideas?

I am pretty sure there is something very cool (like RCU) which allows 
avoiding locks in this situation, I am just too ignorant and do not know it :)

>> +	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.

Is it some generic rule that errors must always be "int"? I was just told 
that gcc on PPC64 will generate an extra instruction to cut 64bit long to 
32bit int so I am just trying to use "long" everywhere. Very simple but 
still optimization :)

>> +{
>> +	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.

Okay.

>> +
>> +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);
>> +	}
>> +}
>

-- 
Alexey