Re: [RFC] Heterogeneous memory management (mirror process address space on a device mmu).

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, May 6, 2014 at 8:00 AM, Jerome Glisse <j.glisse@gmail.com> wrote:
>
> So question becomes how to implement process address space mirroring
> without pinning memory and track cpu page table update knowing that
> device page table update is unbound can not be atomic from cpu point
> of view.

Perhaps as a fake TLB and interacting with the TLB shootdown? And
making sure that everything is atomic?

Some of these devices are going to actually *share* the real page
tables. Not "cache" them. Actually use the page tables directly.
That's where all these on-die APU things are going, where the device
really ends up being something much more like ASMP (asymmetric
multi-processing) than a traditional external device.

So we *will* have to extend our notion of TLB shootdown to have not
just a mask of possible active CPU's, but possible active devices. No
question about that.

But doing this with sleeping in some stupid VM notifier is completely
out of the question, because it *CANNOT EVEN WORK* for that eventual
real goal of sharing the physical page tables where the device can do
things like atomic dirty/accessed bit settings etc. It can only work
for crappy sh*t that does the half-way thing. It's completely racy wrt
the actual page table updates. That kind of page table sharing needs
true atomicity for exactly the same reason we need it for our current
SMP. So it needs to have all the same page table locking rules etc.
Not that shitty notifier callback.

As I said, the VM notifiers were misdesigned to begin with. They are
an abomination. We're not going to extend on that and make it worse.
We are *certainly* not going to make them blocking and screwing our
core VM that way. And that's doubly and triply true when it cannot
work for the generic case _anyway_.

              Linus

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