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Re: [Qemu-ppc] [PATCH 2/2] vfio-powerpc: added VFIO support

From: Benjamin Herrenschmidt
Subject: Re: [Qemu-ppc] [PATCH 2/2] vfio-powerpc: added VFIO support
Date: Wed, 11 Jul 2012 07:53:58 +1000

On Tue, 2012-07-10 at 15:48 -0600, Alex Williamson wrote:
> > specific hypervisor calls, not sure what a MemoryListener would do
> > here ...
> Hmm, the guest directed iommu updates via hypercalls may not really fit
> the MemoryListener model.  I'm just trying to think of ways to avoid
> having an offshoot of vfio in the power code base by making use of
> common abstraction layers.  If we got a region_add/del callback we could
> potentially move the spapr map and unmap into vfio like we do for x86.
> Thanks, 

In the end we don't really want to use that anyway. map and unmap are
*extremely* performance sensitive in practice, so plan is to implement
the hypercall directly in the kernel KVM at a level where it won't even
go near generic code :-)

Basically, when the hypercall gets in, we take control in what we call
"real mode" on powerpc (MMU off, translation disabled), we have a window
to implement critical stuff like this before we context switch the MMU
to the host context (which on P7 is quite expensive).

This is where I want to go directly whack the TCE table as used by the
HW (provided the page has a good PTE of course), pretty much like we do
for populating the main MMU hash table.

So the map/unmap path will be entirely in arch specific code. The one
you see in Alexey code is basically only ever going to be used by
something like qemu full emulation...


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