Subject: Re: [PATCH kernel v9 28/32] powerpc/mmu: Add userspace-to-physical addresses translation cache

From: David Gibson <[email-masked]>
Date: Thu, 30 Apr 2015 16:34:55 +1000

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