[PATCH v2 00/13] KVM: s390: pv: implement lazy destroy

Claudio Imbrenda
2021-07-28T14:26:53Z

Previously, when a protected VM was rebooted or when it was shut down,
its memory was made unprotected, and then the protected VM itself was
destroyed. Looping over the whole address space can take some time,
considering the overhead of the various Ultravisor Calls (UVCs). This
means that a reboot or a shutdown would take a potentially long amount
of time, depending on the amount of used memory.

This patchseries implements a deferred destroy mechanism for protected
guests. When a protected guest is destroyed, its memory is cleared in
background, allowing the guest to restart or terminate significantly
faster than before.

There are 2 possibilities when a protected VM is torn down:
* it still has an address space associated (reboot case)
* it does not have an address space anymore (shutdown case)

For the reboot case, the reference count of the mm is increased, and
then a background thread is started to clean up. Once the thread went
through the whole address space, the protected VM is actually
destroyed.

For the shutdown case, a list of pages to be destroyed is formed when
the mm is torn down. Instead of just unmapping the pages when the
address space is being torn down, they are also set aside. Later when
KVM cleans up the VM, a thread is started to clean up the pages from
the list.

This means that the same address space can have memory belonging to
more than one protected guest, although only one will be running, the
others will in fact not even have any CPUs.

When a guest is destroyed, its memory still counts towards its memory
control group until it's actually freed (I tested this experimentally)

When the system runs out of memory, if a guest has terminated and its
memory is being cleaned asynchronously, the OOM killer will wait a
little and then see if memory has been freed. This has the practical
effect of slowing down memory allocations when the system is out of
memory to give the cleanup thread time to cleanup and free memory, and
avoid an actual OOM situation.

v1->v2
* rebased on a more recent kernel
* improved/expanded some patch descriptions
* improves/expanded some comments
* added patch 1, which prevents stall notification when the system is
under heavy load.
* rename some members of struct deferred_priv to improve readability
* avoid an use-after-free bug of the struct mm in case of shutdown
* add missing return when lazy destroy is disabled
* add support for OOM notifier

Claudio Imbrenda (13):
KVM: s390: pv: avoid stall notifications for some UVCs
KVM: s390: pv: leak the ASCE page when destroy fails
KVM: s390: pv: properly handle page flags for protected guests
KVM: s390: pv: handle secure storage violations for protected guests
KVM: s390: pv: handle secure storage exceptions for normal guests
KVM: s390: pv: refactor s390_reset_acc
KVM: s390: pv: usage counter instead of flag
KVM: s390: pv: add export before import
KVM: s390: pv: lazy destroy for reboot
KVM: s390: pv: extend lazy destroy to handle shutdown
KVM: s390: pv: module parameter to fence lazy destroy
KVM: s390: pv: add OOM notifier for lazy destroy
KVM: s390: pv: add support for UV feature bits

arch/s390/include/asm/gmap.h | 5 +-
arch/s390/include/asm/mmu.h | 3 +
arch/s390/include/asm/mmu_context.h | 2 +
arch/s390/include/asm/pgtable.h | 16 +-
arch/s390/include/asm/uv.h | 26 ++-
arch/s390/kernel/uv.c | 144 ++++++++++++++-
arch/s390/kvm/kvm-s390.c | 6 +-
arch/s390/kvm/kvm-s390.h | 2 +-
arch/s390/kvm/pv.c | 272 ++++++++++++++++++++++++++--
arch/s390/mm/fault.c | 22 ++-
arch/s390/mm/gmap.c | 86 ++++++---
11 files changed, 530 insertions(+), 54 deletions(-)

--
2.31.1