Re: [PATCH 3/3] PPC: KVM: Add support for 64bit TCE windows

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2014-06-05 at 13:56 +0200, Alexander Graf wrote:
> What if we ask user space to give us a pointer to user space allocated 
> memory along with the TCE registration? We would still ask user space to 
> only use the returned fd for TCE modifications, but would have some 
> nicely swappable memory we can store the TCE entries in.

That isn't going to work terribly well for VFIO :-) But yes, for
emulated devices, we could improve things a bit, including for
the 32-bit TCE tables.

For emulated, the real mode path could walk the page tables and fallback
to virtual mode & get_user if the page isn't present, thus operating
directly on qemu memory TCE tables instead of the current pinned stuff.

However that has a cost in performance, but since that's really only
used for emulated devices and PAPR VIOs, it might not be a huge issue.

But for VFIO we don't have much choice, we need to create something the
HW can access.

> In fact, the code as is today can allocate an arbitrary amount of pinned 
> kernel memory from within user space without any checks.

Right. We should at least account it in the locked limit.

Cheers,
Ben.