Subject: Re: [PATCH kernel v9 27/32] powerpc/iommu/ioda2: Add get_table_size() to calculate the size of future table

From: Alexey Kardashevskiy <[email-masked]>
Date: Fri, 01 May 2015 16:53:08 +1000

On 05/01/2015 03:12 PM, David Gibson wrote:
> On Fri, May 01, 2015 at 02:10:58PM +1000, Alexey Kardashevskiy wrote:
>> On 04/29/2015 04:40 PM, David Gibson wrote:
>>> On Sat, Apr 25, 2015 at 10:14:51PM +1000, Alexey Kardashevskiy wrote:
>>>> This adds a way for the IOMMU user to know how much a new table will
>>>> use so it can be accounted in the locked_vm limit before allocation
>>>> happens.
>>>>
>>>> This stores the allocated table size in pnv_pci_create_table()
>>>> so the locked_vm counter can be updated correctly when a table is
>>>> being disposed.
>>>>
>>>> This defines an iommu_table_group_ops callback to let VFIO know
>>>> how much memory will be locked if a table is created.
>>>>
>>>> Signed-off-by: Alexey Kardashevskiy <[email-masked]>
>>>> ---
>>>> Changes:
>>>> v9:
>>>> * reimplemented the whole patch
>>>> ---
>>>>   arch/powerpc/include/asm/iommu.h          |  5 +++++
>>>>   arch/powerpc/platforms/powernv/pci-ioda.c | 14 ++++++++++++
>>>>   arch/powerpc/platforms/powernv/pci.c      | 36 +++++++++++++++++++++++++++++++
>>>>   arch/powerpc/platforms/powernv/pci.h      |  2 ++
>>>>   4 files changed, 57 insertions(+)
>>>>
>>>> diff --git a/arch/powerpc/include/asm/iommu.h b/arch/powerpc/include/asm/iommu.h
>>>> index 1472de3..9844c106 100644
>>>> --- a/arch/powerpc/include/asm/iommu.h
>>>> +++ b/arch/powerpc/include/asm/iommu.h
>>>> @@ -99,6 +99,7 @@ struct iommu_table {
>>>>   	unsigned long  it_size;      /* Size of iommu table in entries */
>>>>   	unsigned long  it_indirect_levels;
>>>>   	unsigned long  it_level_size;
>>>> +	unsigned long  it_allocated_size;
>>>>   	unsigned long  it_offset;    /* Offset into global table */
>>>>   	unsigned long  it_base;      /* mapped address of tce table */
>>>>   	unsigned long  it_index;     /* which iommu table this is */
>>>> @@ -155,6 +156,10 @@ extern struct iommu_table *iommu_init_table(struct iommu_table * tbl,
>>>>   struct iommu_table_group;
>>>>
>>>>   struct iommu_table_group_ops {
>>>> +	unsigned long (*get_table_size)(
>>>> +			__u32 page_shift,
>>>> +			__u64 window_size,
>>>> +			__u32 levels);
>>>>   	long (*create_table)(struct iommu_table_group *table_group,
>>>>   			int num,
>>>>   			__u32 page_shift,
>>>> diff --git a/arch/powerpc/platforms/powernv/pci-ioda.c b/arch/powerpc/platforms/powernv/pci-ioda.c
>>>> index e0be556..7f548b4 100644
>>>> --- a/arch/powerpc/platforms/powernv/pci-ioda.c
>>>> +++ b/arch/powerpc/platforms/powernv/pci-ioda.c
>>>> @@ -2062,6 +2062,18 @@ static void pnv_pci_ioda2_setup_bypass_pe(struct pnv_phb *phb,
>>>>   }
>>>>
>>>>   #ifdef CONFIG_IOMMU_API
>>>> +static unsigned long pnv_pci_ioda2_get_table_size(__u32 page_shift,
>>>> +		__u64 window_size, __u32 levels)
>>>> +{
>>>> +	unsigned long ret = pnv_get_table_size(page_shift, window_size, levels);
>>>> +
>>>> +	if (!ret)
>>>> +		return ret;
>>>> +
>>>> +	/* Add size of it_userspace */
>>>> +	return ret + (window_size >> page_shift) * sizeof(unsigned long);
>>>
>>> This doesn't make much sense.  The userspace view can't possibly be a
>>> property of the specific low-level IOMMU model.
>>
>>
>> This it_userspace thing is all about memory preregistration.
>>
>> I need some way to track how many actual mappings the
>> mm_iommu_table_group_mem_t has in order to decide whether to allow
>> unregistering or not.
>>
>> When I clear TCE, I can read the old value which is host physical address
>> which I cannot use to find the preregistered region and adjust the mappings
>> counter; I can only use userspace addresses for this (not even guest
>> physical addresses as it is VFIO and probably no KVM).
>>
>> So I have to keep userspace addresses somewhere, one per IOMMU page, and the
>> iommu_table seems a natural place for this.
>
> Well.. sort of.  But as noted elsewhere this pulls VFIO specific
> constraints into a platform code structure.  And whether you get this
> table depends on the platform IOMMU type rather than on what VFIO
> wants to do with it, which doesn't make sense.
>
> What might make more sense is an opaque pointer io iommu_table for use
> by the table "owner" (in the take_ownership sense).  The pointer would
> be stored in iommu_table, but VFIO is responsible for populating and
> managing its contents.
>
> Or you could just put the userspace mappings in the container.
> Although you might want a different data structure in that case.

Nope. I need this table in in-kernel acceleration to update the mappings 
counter per mm_iommu_table_group_mem_t. In KVM's real mode handlers, I only 
have IOMMU tables, not containers or groups. QEMU creates a guest view of 
the table (KVM_CREATE_SPAPR_TCE) specifying a LIOBN, and then attaches TCE 
tables to it via set of ioctls (one per IOMMU group) to VFIO KVM device.

So if I call it it_opaque (instead of it_userspace), I will still need a 
common place (visible to VFIO and PowerKVM) for this to put:
#define IOMMU_TABLE_USERSPACE_ENTRY(tbl, entry)

So far this place was arch/powerpc/include/asm/iommu.h and the iommu_table 
struct.

> The other thing to bear in mind is that registered regions are likely
> to be large contiguous blocks in user addresses, though obviously not
> contiguous in physical addr.  So you might be able to compaticfy this
> information by storing it as a list of variable length blocks in
> userspace address space, rather than a per-page address..

It is 8 bytes per system page - 8/65536 = 0.00012 (or 26MB for 200GB guest) 
- very little overhead.

> But.. isn't there a bigger problem here.  As Paulus was pointing out,
> there's nothing guaranteeing the page tables continue to contain the
> same page as was there at gup() time.

This can happen if the userspace remaps memory which it registered/mapped 
for DMA via VFIO, no? If so, then the userspace just should not do this, it 
is DMA, it cannot be moved like this. What am I missing here?

> What's going to happen if you REGISTER a memory region, then mremap()
> over it?

The registered pages will remain pinned and PUT_TCE will use that region 
for translation (and this will fail as the userspace addresses changed).

I do not see how it is different from the situation when the userspace 
mapped a page and mremap()ed it while it is DMA-mapped.

> Then attempt to PUT_TCE a page in the region? Or what if you
> mremap() it to someplace else then try to PUT_TCE a page there?

This will fail - a new userspace address has to be preregistered.

> Or REGISTER it again in its new location?

It will be pinned twice + some memory overhead to store the same host 
physical address(es) twice.

-- 
Alexey