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

From: David Gibson <[email-masked]>
Date: Fri, 1 May 2015 14:23:19 +1000

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 <[email-masked]>
> >>---
> >>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.

> > 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?

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

[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).

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.

> >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.

-- 
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