[PATCH v5 00/14] KVM: s390: pv: implement lazy destroy for reboot

Claudio Imbrenda
2021-09-20T13:25:12Z

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.

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.

The shutdown case is more controversial, and it will be dealt with in a
future patchseries.

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

v4->v5
* fixed and improved some patch descriptions
* added some comments to better explain what's going on
* use vma_lookup instead of find_vma
* rename is_protected to protected_count since now it's used as a counter

v3->v4
* added patch 2
* split patch 3
* removed the shutdown part -- will be a separate patchseries
* moved the patch introducing the module parameter

v2->v3
* added definitions for CC return codes for the UVC instruction
* improved make_secure_pte:
- renamed rc to cc
- added comments to explain why returning -EAGAIN is ok
* fixed kvm_s390_pv_replace_asce and kvm_s390_pv_remove_old_asce:
- renamed
- added locking
- moved to gmap.c
* do proper error management in do_secure_storage_access instead of
trying again hoping to get a different exception
* fix outdated patch descriptions

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 (14):
KVM: s390: pv: add macros for UVC CC values
KVM: s390: pv: avoid double free of sida page
KVM: s390: pv: avoid stalls for kvm_s390_pv_init_vm
KVM: s390: pv: avoid stalls when making pages secure
KVM: s390: pv: leak the topmost page table 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: module parameter to fence lazy destroy
KVM: s390: pv: lazy destroy for reboot
KVM: s390: pv: avoid export before import if possible

arch/s390/include/asm/gmap.h | 6 +-
arch/s390/include/asm/mmu.h | 2 +-
arch/s390/include/asm/mmu_context.h | 2 +-
arch/s390/include/asm/pgtable.h | 11 +-
arch/s390/include/asm/uv.h | 16 ++-
arch/s390/kernel/uv.c | 118 ++++++++++++++++--
arch/s390/kvm/intercept.c | 5 +
arch/s390/kvm/kvm-s390.c | 6 +-
arch/s390/kvm/kvm-s390.h | 2 +-
arch/s390/kvm/pv.c | 187 ++++++++++++++++++++++++----
arch/s390/mm/fault.c | 20 ++-
arch/s390/mm/gmap.c | 141 +++++++++++++++++----
12 files changed, 447 insertions(+), 69 deletions(-)

--
2.31.1