Re: [PATCH kernel v9 26/32] powerpc/iommu: Add userspace view of TCE table

From: Alexey Kardashevskiy
Date: Fri May 01 2015 - 03:13:04 EST

On 05/01/2015 02:23 PM, David Gibson wrote:
On Fri, May 01, 2015 at 02:01:17PM +1000, Alexey Kardashevskiy wrote:
On 04/29/2015 04:31 PM, David Gibson wrote:
On Sat, Apr 25, 2015 at 10:14:50PM +1000, Alexey Kardashevskiy wrote:
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>
---
Changes:
v9:
* fixed code flow in error cases added in v8

v8:
* added ENOMEM on failed vzalloc()
---
arch/powerpc/include/asm/iommu.h | 6 ++++++
arch/powerpc/kernel/iommu.c | 18 ++++++++++++++++++
arch/powerpc/platforms/powernv/pci-ioda.c | 22 ++++++++++++++++++++--
3 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/iommu.h b/arch/powerpc/include/asm/iommu.h
index 7694546..1472de3 100644
--- a/arch/powerpc/include/asm/iommu.h
+++ b/arch/powerpc/include/asm/iommu.h
@@ -111,9 +111,15 @@ struct iommu_table {
unsigned long *it_map; /* A simple allocation bitmap for now */
unsigned long it_page_shift;/* table iommu page size */
struct iommu_table_group *it_table_group;
+ unsigned long *it_userspace; /* userspace view of the table */

A single unsigned long doesn't seem like enough.

Why single? This is an array.

As in single per page.


Sorry, I am not following you here.
It is per IOMMU page. MAP/UNMAP work with IOMMU pages which are fully backed with either system page or a huge page.



How do you know
which process's address space this address refers to?

It is a current task. Multiple userspaces cannot use the same container/tables.

Where is that enforced?


It is accessed from VFIO DMA map/unmap which are ioctls() to a container's fd which is per a process. Same for KVM - when it registers IOMMU groups in KVM, fd's of opened IOMMU groups are passed there. Or I did not understand the question...


More to the point, that's a VFIO constraint, but it's here affecting
the design of a structure owned by the platform code.

Right. But keeping in mind KVM, I cannot think of any better design here.


[snip]
static void pnv_pci_ioda_setup_opal_tce_kill(struct pnv_phb *phb,
@@ -2062,12 +2071,21 @@ static long pnv_pci_ioda2_create_table(struct iommu_table_group *table_group,
int nid = pe->phb->hose->node;
__u64 bus_offset = num ? pe->tce_bypass_base : 0;
long ret;
+ unsigned long *uas, uas_cb = sizeof(*uas) * (window_size >> page_shift);
+
+ uas = vzalloc(uas_cb);
+ if (!uas)
+ return -ENOMEM;

I don't see why this is allocated both here as well as in
take_ownership.

Where else? The only alternative is vfio_iommu_spapr_tce but I really do not
want to touch iommu_table fields there.

Well to put it another way, why isn't take_ownership calling create
itself (or at least a common helper).

I am trying to keep DDW stuff away from platform-oriented arch/powerpc/kernel/iommu.c which main purpose is to implement iommu_alloc()&co. It already has

I'd rather move it_userspace allocation completely to vfio_iommu_spapr_tce (should have done earlier, actually), would this be ok?


Clearly the it_userspace table needs to have lifetime which matches
the TCE table itself, so there should be a single function that marks
the beginning of that joint lifetime.


No. it_userspace lives as long as the platform code does not control the table. For IODA2 it is equal for the lifetime of the table, for IODA1/P5IOC2 it is not.



Isn't this function used for core-kernel users of the
iommu as well, in which case it shouldn't need the it_userspace.


No. This is an iommu_table_group_ops callback which calls what the platform
code calls (pnv_pci_create_table()) plus allocates this it_userspace thing.
The callback is only called from VFIO.

Ok.

As touched on above it seems more like this should be owned by VFIO
code than the platform code.

Agree now :) I'll move the allocation to VFIO. Thanks!


--
Alexey
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