[PATCH v2 0/9] KVM: x86/MMU: Optimize disabling dirty logging

Ben Gardon
2022-03-21T22:48:20Z

Currently disabling dirty logging with the TDP MMU is extremely slow.
On a 96 vCPU / 96G VM it takes ~256 seconds to disable dirty logging
with the TDP MMU, as opposed to ~4 seconds with the legacy MMU. This
series optimizes TLB flushes and introduces in-place large page
promotion, to bring the disable dirty log time down to ~3 seconds.

Testing:
Ran KVM selftests and kvm-unit-tests on an Intel Haswell. This
series introduced no new failures.

Performance:

Without this series, TDP MMU:
> ./dirty_log_perf_test -v 96 -s anonymous_hugetlb_1gb
Test iterations: 2
Testing guest mode: PA-bits:ANY, VA-bits:48, 4K pages
guest physical test memory offset: 0x3fe7c0000000
Populate memory time: 4.972184425s
Enabling dirty logging time: 0.001943807s

Iteration 1 dirty memory time: 0.061862112s
Iteration 1 get dirty log time: 0.001416413s
Iteration 1 clear dirty log time: 1.417428057s
Iteration 2 dirty memory time: 0.664103656s
Iteration 2 get dirty log time: 0.000676724s
Iteration 2 clear dirty log time: 1.149387201s
Disabling dirty logging time: 256.682188868s
Get dirty log over 2 iterations took 0.002093137s. (Avg 0.001046568s/iteration)
Clear dirty log over 2 iterations took 2.566815258s. (Avg 1.283407629s/iteration)

Without this series, Legacy MMU:
> ./dirty_log_perf_test -v 96 -s anonymous_hugetlb_1gb
Test iterations: 2
Testing guest mode: PA-bits:ANY, VA-bits:48, 4K pages
guest physical test memory offset: 0x3fe7c0000000
Populate memory time: 4.892940915s
Enabling dirty logging time: 0.001864603s

Iteration 1 dirty memory time: 0.060490391s
Iteration 1 get dirty log time: 0.001416277s
Iteration 1 clear dirty log time: 0.323548614s
Iteration 2 dirty memory time: 29.217064826s
Iteration 2 get dirty log time: 0.000696202s
Iteration 2 clear dirty log time: 0.907089084s
Disabling dirty logging time: 4.246216551s
Get dirty log over 2 iterations took 0.002112479s. (Avg 0.001056239s/iteration)
Clear dirty log over 2 iterations took 1.230637698s. (Avg 0.615318849s/iteration)

With this series, TDP MMU:
(Updated since RFC. Pulling out patches 1-4 could have a performance impact.)
> ./dirty_log_perf_test -v 96 -s anonymous_hugetlb_1gb
Test iterations: 2
Testing guest mode: PA-bits:ANY, VA-bits:48, 4K pages
guest physical test memory offset: 0x3fe7c0000000
Populate memory time: 4.878083336s
Enabling dirty logging time: 0.001874340s

Iteration 1 dirty memory time: 0.054867383s
Iteration 1 get dirty log time: 0.001368377s
Iteration 1 clear dirty log time: 1.406960856s
Iteration 2 dirty memory time: 0.679301083s
Iteration 2 get dirty log time: 0.000662905s
Iteration 2 clear dirty log time: 1.138263359s
Disabling dirty logging time: 3.169381810s
Get dirty log over 2 iterations took 0.002031282s. (Avg 0.001015641s/iteration)
Clear dirty log over 2 iterations took 2.545224215s. (Avg 1.272612107s/iteration)

Patch breakdown:
Patches 1-4 remove the need for a vCPU pointer to make_spte
Patches 5-8 are small refactors in preparation for in-place lpage promotion
Patch 9 implements in-place largepage promotion when disabling dirty logging

Changelog:
RFC -> v1:
Dropped the first 4 patches from the series. Patch 1 was sent
separately, patches 2-4 will be taken over by Sean Christopherson.
Incorporated David Matlack's Reviewed-by.
v1 -> v2:
Several patches were queued and dropped from this revision.
Incorporated feedback from Peter Xu on the last patch in the series.
Refreshed performance data
Between versions 1 and 2 of this series, disable time without
the TDP MMU went from 45s to 256, a major regression. I was
testing on a skylake before and haswell this time, but that
does not explain the huge performance loss.

Ben Gardon (9):
KVM: x86/mmu: Move implementation of make_spte to a helper
KVM: x86/mmu: Factor mt_mask out of __make_spte
KVM: x86/mmu: Factor shadow_zero_check out of __make_spte
KVM: x86/mmu: Replace vcpu argument with kvm pointer in make_spte
KVM: x86/mmu: Factor out the meat of reset_tdp_shadow_zero_bits_mask
KVM: x86/mmu: Factor out part of vmx_get_mt_mask which does not depend
on vcpu
KVM: x86/mmu: Add try_get_mt_mask to x86_ops
KVM: x86/mmu: Make kvm_is_mmio_pfn usable outside of spte.c
KVM: x86/mmu: Promote pages in-place when disabling dirty logging

arch/x86/include/asm/kvm-x86-ops.h | 1 +
arch/x86/include/asm/kvm_host.h | 2 +
arch/x86/kvm/mmu/mmu.c | 21 +++++----
arch/x86/kvm/mmu/mmu_internal.h | 6 +++
arch/x86/kvm/mmu/spte.c | 39 +++++++++++-----
arch/x86/kvm/mmu/spte.h | 6 +++
arch/x86/kvm/mmu/tdp_mmu.c | 73 +++++++++++++++++++++++++++++-
arch/x86/kvm/svm/svm.c | 9 ++++
arch/x86/kvm/vmx/vmx.c | 25 ++++++++--
9 files changed, 155 insertions(+), 27 deletions(-)

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2.35.1.894.gb6a874cedc-goog