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, ¤t->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
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