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

David Gibson
2015-05-01T03:56:03Z

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On Thu, Apr 30, 2015 at 06:25:25PM +1000, Paul Mackerras wrote:
> On Thu, Apr 30, 2015 at 04:34:55PM +1000, David Gibson wrote:
> > 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.
>
> We can access vmalloc'd arrays in real mode using real_vmalloc_addr().

Ah, ok.

> > I can't think of a circumstance in which you can use hpas where you
> > couldn't just walk the page tables anyway.
>
> The problem with walking the page tables is that there is no guarantee
> that the page you find that way is the page that was returned by the
> gup_fast() we did earlier. Storing the hpas means that we know for
> sure that the page we're doing DMA to is one that we have an elevated
> page count on.
>
> Also, there are various points where a Linux PTE is made temporarily
> invalid for a short time. If we happened to do a H_PUT_TCE on one cpu
> while another cpu was doing that, we'd get a spurious failure returned
> by the H_PUT_TCE.

I think we want this explanation in the commit message. Anr/or in a
comment somewhere, I'm not sure.

--
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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