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.

[snip]

+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;
+	}

So, I've thought more about this and I'm really confused as to what
this is supposed to be accomplishing.

I see that you need to keep track of what regions are registered, so
you don't double lock or unlock, but I don't see what the point of
actualy storing the translations in hpas is.

I had assumed it was so that you could later on get to the
translations in real mode when you do in-kernel acceleration.  But
that doesn't make sense, because the array is vmalloc()ed, so can't be
accessed in real mode anyway.

I can't think of a circumstance in which you can use hpas where you
couldn't just walk the page tables anyway.

-- 
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