[0/3] KVM: Split huge pages mapped by the TDP MMU on fault

Message

David Matlack April 1, 2022, 11:37 p.m. UTC

Now that the TDP MMU has a mechanism to split huge pages, it is trivial
to use it in the fault path whenever a huge page needs to be replaced
with a mapping at a lower level.

The main beneficiary of this change is NX HugePages, which now no longer
unmaps huge pages when instructions are fetched from them. This means
that the VM does not have to take faults when accessing other parts of
the huge page (for whatever reason). The other beneficiary of this
change is workloads that set eage_page_split=N, which can be desirable
for read-heavy workloads. The performance impacts are discussed in more
detail in PATCH 3.

To validate the performance impact on NX HugePages, this series
introduces a new selftest that leverages perf_test_util.c to execute
from a given region of memory across a variable number of vCPUS.

This new selftest, while only 188 lines, is largely a whittled down
copy-paste of access_tracking_perf_test.c. So there is obviously some
room for improvement and refactoring that I am procrastinating on.

Tested: Ran all kvm-unit-tests and KVM selftests.

David Matlack (3):
  KVM: selftests: Introduce a selftest to measure execution performance
  KVM: x86/mmu: Pass account_nx to tdp_mmu_split_huge_page()
  KVM: x86/mmu: Split huge pages mapped by the TDP MMU on fault

 arch/x86/kvm/mmu/tdp_mmu.c                    |  44 ++--
 tools/testing/selftests/kvm/.gitignore        |   1 +
 tools/testing/selftests/kvm/Makefile          |   1 +
 .../testing/selftests/kvm/execute_perf_test.c | 188 ++++++++++++++++++
 .../selftests/kvm/include/perf_test_util.h    |   2 +
 .../selftests/kvm/lib/perf_test_util.c        |  25 ++-
 6 files changed, 244 insertions(+), 17 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/execute_perf_test.c


base-commit: d1fb6a1ca3e535f89628193ab94203533b264c8c