Re: [PATCH kernel v10 33/34] vfio: powerpc/spapr: Register memory and define IOMMU v2

From: Alexey Kardashevskiy
Date: Thu May 14 2015 - 02:09:16 EST

On 05/14/2015 07:30 AM, Alex Williamson wrote:
On Tue, 2015-05-12 at 01:39 +1000, Alexey Kardashevskiy wrote:
The existing implementation accounts the whole DMA window in
the locked_vm counter. This is going to be worse with multiple
containers and huge DMA windows. Also, real-time accounting would requite
additional tracking of accounted pages due to the page size difference -
IOMMU uses 4K pages and system uses 4K or 64K pages.

Another issue is that actual pages pinning/unpinning happens on every
DMA map/unmap request. This does not affect the performance much now as
we spend way too much time now on switching context between
guest/userspace/host but this will start to matter when we add in-kernel
DMA map/unmap acceleration.

This introduces a new IOMMU type for SPAPR - VFIO_SPAPR_TCE_v2_IOMMU.
2 new ioctls to register/unregister DMA memory -
which receive user space address and size of a memory region which
needs to be pinned/unpinned and counted in locked_vm.
New IOMMU splits physical pages pinning and TCE table update
into 2 different operations. It requires:
1) guest pages to be registered first
2) consequent map/unmap requests to work only with pre-registered memory.
For the default single window case this means that the entire guest
(instead of 2GB) needs to be pinned before using VFIO.
When a huge DMA window is added, no additional pinning will be
required, otherwise it would be guest RAM + 2GB.

The new memory registration ioctls are not supported by
VFIO_SPAPR_TCE_IOMMU. Dynamic DMA window and in-kernel acceleration
will require memory to be preregistered in order to work.

The accounting is done per the user process.

This advertises v2 SPAPR TCE IOMMU and restricts what the userspace
can do with v1 or v2 IOMMUs.

In order to support memory pre-registration, we need a way to track
the use of every registered memory region and only allow unregistration
if a region is not in use anymore. So we need a way to tell from what
region the just cleared TCE was from.

This adds a userspace view of the TCE table into iommu_table struct.
It contains userspace address, one per TCE entry. The table is only
allocated when the ownership over an IOMMU group is taken which means
it is only used from outside of the powernv code (such as VFIO).

Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxxxx>
[aw: for the vfio related changes]
Acked-by: Alex Williamson <alex.williamson@xxxxxxxxxx>

Alex, should I remove your "acked-by" in the cases like this and
get another one?

Generally if it's more than a trivial change, you'll want fresh acks.

* moved it_userspace allocation to vfio_iommu_spapr_tce as it VFIO
specific thing
* squashed "powerpc/iommu: Add userspace view of TCE table" into this as
it is
a part of IOMMU v2
* s/tce_iommu_use_page_v2/tce_iommu_prereg_ua_to_hpa/
* fixed some function names to have "tce_iommu_" in the beginning rather
just "tce_"
* as mm_iommu_mapped_inc() can now fail, check for the return code

* s/tce_get_hva_cached/tce_iommu_use_page_v2/

* now memory is registered per mm (i.e. process)
* moved memory registration code to powerpc/mmu
* merged "vfio: powerpc/spapr: Define v2 IOMMU" into this
* limited new ioctls to v2 IOMMU
* updated doc
* unsupported ioclts return -ENOTTY instead of -EPERM

* tce_get_hva_cached() returns hva via a pointer

* updated docs
* s/kzmalloc/vzalloc/
* in tce_pin_pages()/tce_unpin_pages() removed @vaddr, @size and
replaced offset with index
* renamed vfio_iommu_type_register_memory to vfio_iommu_spapr_register_memory
and removed duplicating vfio_iommu_spapr_register_memory
Documentation/vfio.txt | 31 ++-
arch/powerpc/include/asm/iommu.h | 6 +
drivers/vfio/vfio_iommu_spapr_tce.c | 516 ++++++++++++++++++++++++++++++------
include/uapi/linux/vfio.h | 27 ++
4 files changed, 494 insertions(+), 86 deletions(-)

diff --git a/Documentation/vfio.txt b/Documentation/vfio.txt
index 96978ec..7dcf2b5 100644
--- a/Documentation/vfio.txt
+++ b/Documentation/vfio.txt
@@ -289,10 +289,12 @@ PPC64 sPAPR implementation note

This implementation has some specifics:

-1) Only one IOMMU group per container is supported as an IOMMU group
-represents the minimal entity which isolation can be guaranteed for and
-groups are allocated statically, one per a Partitionable Endpoint (PE)
+1) On older systems (POWER7 with P5IOC2/IODA1) only one IOMMU group per
+container is supported as an IOMMU table is allocated at the boot time,
+one table per a IOMMU group which is a Partitionable Endpoint (PE)
(PE is often a PCI domain but not always).
+Newer systems (POWER8 with IODA2) have improved hardware design which allows
+to remove this limitation and have multiple IOMMU groups per a VFIO container.

2) The hardware supports so called DMA windows - the PCI address range
within which DMA transfer is allowed, any attempt to access address space
@@ -427,6 +429,29 @@ The code flow from the example above should be slightly changed:


+5) There is v2 of SPAPR TCE IOMMU. It deprecates VFIO_IOMMU_ENABLE/
+VFIO_IOMMU_DISABLE and implements 2 new ioctls:
+(which are unsupported in v1 IOMMU).
+PPC64 paravirtualized guests generate a lot of map/unmap requests,
+and the handling of those includes pinning/unpinning pages and updating
+mm::locked_vm counter to make sure we do not exceed the rlimit.
+The v2 IOMMU splits accounting and pinning into separate operations:
+receive a user space address and size of the block to be pinned.
+Bisecting is not supported and VFIO_IOMMU_UNREGISTER_MEMORY is expected to
+be called with the exact address and size used for registering
+the memory block. The userspace is not expected to call these often.
+The ranges are stored in a linked list in a VFIO container.
+- VFIO_IOMMU_MAP_DMA/VFIO_IOMMU_UNMAP_DMA ioctls only update the actual
+IOMMU table and do not do pinning; instead these check that the userspace
+address is from pre-registered range.
+This separation helps in optimizing DMA for guests.

[1] VFIO was originally an acronym for "Virtual Function I/O" in its
diff --git a/arch/powerpc/include/asm/iommu.h b/arch/powerpc/include/asm/iommu.h
index c8bad21..763c041 100644
--- a/arch/powerpc/include/asm/iommu.h
+++ b/arch/powerpc/include/asm/iommu.h
@@ -113,10 +113,16 @@ struct iommu_table {
unsigned long it_page_shift;/* table iommu page size */
struct list_head it_group_list;/* List of iommu_table_group_link */
+ unsigned long *it_userspace; /* userspace view of the table */
struct iommu_table_ops *it_ops;

+#define IOMMU_TABLE_USERSPACE_ENTRY(tbl, entry) \
+ ((tbl)->it_userspace ? \
+ &((tbl)->it_userspace[(entry) - (tbl)->it_offset]) : \
/* Pure 2^n version of get_order */
static inline __attribute_const__
int get_iommu_order(unsigned long size, struct iommu_table *tbl)
diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
index 8943b29..e7e8db3 100644
--- a/drivers/vfio/vfio_iommu_spapr_tce.c
+++ b/drivers/vfio/vfio_iommu_spapr_tce.c
@@ -19,8 +19,10 @@
#include <linux/uaccess.h>
#include <linux/err.h>
#include <linux/vfio.h>
+#include <linux/vmalloc.h>
#include <asm/iommu.h>
#include <asm/tce.h>
+#include <asm/mmu_context.h>

#define DRIVER_VERSION "0.1"
#define DRIVER_AUTHOR "aik@xxxxxxxxx"
@@ -81,6 +83,11 @@ static void decrement_locked_vm(long npages)
* into DMA'ble space using the IOMMU

+struct tce_iommu_group {
+ struct list_head next;
+ struct iommu_group *grp;
* The container descriptor supports only a single group per container.
* Required by the API as the container is not supplied with the IOMMU group
@@ -88,11 +95,98 @@ static void decrement_locked_vm(long npages)
struct tce_container {
struct mutex lock;
- struct iommu_group *grp;
bool enabled;
unsigned long locked_pages;
+ bool v2;
+ struct iommu_table *tables[IOMMU_TABLE_GROUP_MAX_TABLES];
+ struct list_head group_list;

You're wasting space by not packing your bools next to each other.

I'll fix it :)


+static long tce_iommu_unregister_pages(struct tce_container *container,
+ __u64 vaddr, __u64 size)
+ long ret;
+ struct mm_iommu_table_group_mem_t *mem;
+ if ((vaddr & ~PAGE_MASK) || (size & ~PAGE_MASK))
+ return -EINVAL;
+ mem = mm_iommu_get(vaddr, size >> PAGE_SHIFT);
+ if (!mem)
+ return -EINVAL;
+ ret = mm_iommu_put(mem); /* undo kref_get() from mm_iommu_get() */
+ if (!ret)
+ ret = mm_iommu_put(mem);

Should \put\ really be able to fail?

tce_iommu_unregister_pages() is called from ioctl so yes, the userspace deserves to know that the memory will remain pinned.

I think you really need to examine
your reference model, mm_iommu_put() looks pretty suspicious. If
there's an implicit reference by being mapped, it should be handled that
way, not via an atomic that gets decremented then corrected.

One implicit reference (*) in @mapped (from atomic_set(&mem->mapped, 1)) is only to protect against the race between checking for active mappings and putting the reference a registered memory descriptor.

If tce_iommu_unregister_pages() is called when @mapped > 1, then EBUSY is returned.

If tce_iommu_unregister_pages() is called when @mapped == 1 or 0, then there is no active mapping, @mapped becomes zero (if it is not already) and we can safely put the descriptor. All consequent mm_iommu_mapped_inc() calls will fail to increment @mapped and return error.

After looking there more, there are 2 bugs though:

--- a/arch/powerpc/mm/mmu_context_hash64_iommu.c
+++ b/arch/powerpc/mm/mmu_context_hash64_iommu.c
@@ -178,9 +178,9 @@ EXPORT_SYMBOL_GPL(mm_iommu_get);

long mm_iommu_put(struct mm_iommu_table_group_mem_t *mem)
- if (1 != atomic_dec_if_positive(&mem->mapped)) {
+ if (atomic_dec_if_positive(&mem->mapped) > 1) {
/* There are mappings, exit */
- atomic_inc(&mem->mapped);
+ atomic_inc_not_zero(&mem->mapped);
return -EBUSY;

s/1!=/1</ is to allow putting second/third/... reference of mem->kref and atomic_inc_not_zero() is to not elevate the counter if another thread managed to release the very last mapping and decrement my implicit reference (*).

Am I still missing something here?

That's not only not atomic, but causes lots of fallout with references that don't
get released.
Notice how you don't even check the return value at the
call location of this function?

Ouch. This is a bug. @ret needs to be returned to the userspace.

How many references does that
potentially leave and where do the get resolved?

Every successful "register" should be coupled with successful "unregister" (if it failed - just repeat). If this did not happen, memory remains pinned till the process exit, and then it is unpinned unconditionally.

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