RE: [RFC v2 1/2] powerpc/pseries/iommu: Share the per-cpu TCE page with the hypervisor.

On Sun, Nov 10, 2019 at 07:40:06PM +0000, David Gibson wrote:

On Fri, Nov 08, 2019 at 03:00:10PM -0800, Ram Pai wrote:

quoted

The hypervisor needs to access the contents of the page holding the TCE
entries while setting up the TCE entries in the IOMMU's TCE table.

For SecureVMs, since this page is encrypted, the hypervisor cannot
access valid entries. Share the page with the hypervisor. This ensures
that the hypervisor sees those valid entries.

Why is this safe?
   The page contains only TCE entries; not any sensitive data
   belonging to the Secure VM. The hypervisor has a genuine need to know
   the value of the TCE entries, without which it will not be able to
   DMA to/from the pages pointed to by the TCE entries. In a Secure
   VM the TCE entries point to pages that are also shared with the
   hypervisor; example: pages containing bounce buffers.

The bit that may not be obvious to reviewers from the above is this:

This is *not* a page of "live" TCEs which are actively used for
translation.  Instead this is just a transient buffer with a batch of
TCEs to set, passed to the hypervisor with the H_PUT_TCE_INDIRECT call.

That is true.  I should have stated that explicitly. Thanks. will
update the commit log.


RP