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

From: David Gibson <[email-masked]>
Date: Tue, 5 May 2015 22:12:51 +1000

On Fri, May 01, 2015 at 09:26:48PM +1000, Alexey Kardashevskiy wrote:
> 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.

I'd suggest putting it as a new field in mm_struct, guarded by a
CONFIG_VFIO_PREREGISTER (or something) which you can make sure is
selected by CONFIG_SPAPR_TCE_IOMMU.

> 
> 
> >>  } 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.

Ok, you convinced me.  And if you have to translate them each from
struct page to hpa at this point, then the gup() in a loop does make
as much sense as anything, so ok.

> 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
> :)

I can't quickly see an answer either, sorry.

> >>+	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 :)

Ok, I guess leave it.  Probably makes little difference either way.

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

-- 
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