tree e5d44b86da10ceb29d2dbbc7af3a0578abd74125
parent 3ee9410a942a598494ca64255b35032ece44ca63
author Sebastian Ene <sebastianene@google.com> 1690371603 +0000
committer Sebastian Ene <sebastianene@google.com> 1725890091 +0000

KVM: arm64: Coalesce host stage2 entries on ownership reclaim

This optimization allows us to re-create higher order block mappings in
the host stage2 pagetables after we teardown a guest VM. The coalescing
code is triggered on host_stage2_set_owner_locked path when we annotate
the entries in the host stage2 page-tables with an invalid entry that has
the owner set to PKVM_ID_HOST. This can also be triggered from
page_relinquish when we do page insertion in the ballooning code.

This patch reintroduces the _post callback walker to be able to zap the
page table entries on ownership re-assignment.

When the host reclaims ownership during guest teardown, the page table
walker drops the refcount of the counted entries and clears out
unreferenced entries (refcount == 1). Clearing out the entry installs a
zero PTE. When the host stage2 receives a data abort because there is no
mapping associated, it will try to create the largest possible block
mapping from the founded leaf entry.
With the current patch, we increase the chances of finding a leaf entry
that has level < 3 if the requested region comes from a reclaimed torned
down VM memory. This has the advantage of reducing the TLB pressure at
host stage2.

To be able to do coalescing, we modify the way we do refcounting by not
counting the following descriptor types at host stage 2:
- non-zero invalid PTEs
- any descriptor that has at least one of the reserved-high bits(58-55)
toogled
- non-default attribute mappings
- page table descriptors

The algorithm works as presented below:

  Is refcount(child(pte_table)) == 1 ?

  Yes -> (because we left only default mappings)
	Zap the table by setting 0 in the pte_table
	and put the page that holds the level 3 entries
	back into the memcache

  level 2
+---------+
|         |
|  ...    |
| pte_table---+      level 3      -> we can now re-create a 2Mb mapping
|  ...    |   +---> +---------+
|         |         |         |
|         |         |         |
|         |         |def entry|
+---------+         |         |
                    |def entry|
                    |         |
                    |  ...    |
                    +---------+

Change-Id: I983372a0fac876336ee6799be1f3c4a50015880c
Signed-off-by: Sebastian Ene <sebastianene@google.com>