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[v3,0/2] KVM: Split huge pages mapped by the TDP MMU on fault

Message ID 20221109185905.486172-1-dmatlack@google.com (mailing list archive)
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Series KVM: Split huge pages mapped by the TDP MMU on fault | expand

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David Matlack Nov. 9, 2022, 6:59 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 is short but contains a lot of duplicate code with
access_tracking_perf_test.c, and the changes to perf_test_util.c to
support execution are a bit gross. I'd be fine with deferring this test
to a later series, but I want to include it here to demonstrate how I
tested the TDP MMU patch.

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

Cc: Mingwei Zhang <mizhang@google.com>

v3:
 - Rebased on top of latest kvm/queue.
 - R-b from Mingwei.
 - Add comment about sp initialization [Mingwei]

v2: https://lore.kernel.org/kvm/20221019234050.3919566-1-dmatlack@google.com/
 - Rebased on top of Sean's precise NX series
   (https://lore.kernel.org/kvm/20221019165618.927057-1-seanjc@google.com/)

David Matlack (2):
  KVM: selftests: Introduce a selftest to measure execution performance
  KVM: x86/mmu: Split huge pages mapped by the TDP MMU on fault

 arch/x86/kvm/mmu/tdp_mmu.c                    |  73 ++++---
 tools/testing/selftests/kvm/.gitignore        |   1 +
 tools/testing/selftests/kvm/Makefile          |   1 +
 .../testing/selftests/kvm/execute_perf_test.c | 185 ++++++++++++++++++
 .../selftests/kvm/include/perf_test_util.h    |   2 +
 .../selftests/kvm/lib/perf_test_util.c        |  25 ++-
 6 files changed, 247 insertions(+), 40 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/execute_perf_test.c


base-commit: d663b8a285986072428a6a145e5994bc275df994