[PATCH 0/9] x86/kvm/nVMX: optimize MMU switch between L1 and L2

Vitaly Kuznetsov
2018-08-02T10:01:23Z

Major changes since RFC:
- With multi-root caching we shouldn't worry about CR3/EPTP changes any
more, when MMU is split we have two separate caches and these should
work well.
- Merge 'scache' data with kvm_mmu_page_role to avoid data and code
duplication.

Currently, when we switch from L1 to L2 (VMX) we do the following:
- Re-initialize L1 MMU as shadow EPT MMU (nested_ept_init_mmu_context())
- Re-initialize 'nested' MMU (nested_vmx_load_cr3() -> init_kvm_nested_mmu())

When we switch back we do:
- Re-initialize L1 MMU (nested_vmx_load_cr3() -> init_kvm_tdp_mmu())

This seems to be sub-optimal. Initializing MMU is expensive (thanks to
update_permission_bitmask(), update_pkru_bitmask(),..) Try solving the
issue by splitting L1-normal and L1-nested MMUs and checking if MMU reset
is really needed. This spares us about 1000 cpu cycles on nested vmexit.

Brief look at SVM makes me think it can be optimized the exact same way.
I'll do this in a separate series if nobody objects.

Paolo Bonzini (1):
x86/kvm/mmu: get rid of redundant kvm_mmu_setup()

Vitaly Kuznetsov (8):
x86/kvm/mmu: make vcpu->mmu a pointer to the current MMU
x86/kvm/mmu.c: set get_pdptr hook in kvm_init_shadow_ept_mmu()
x86/kvm/mmu.c: add kvm_mmu parameter to kvm_mmu_free_roots()
x86/kvm/mmu: introduce guest_mmu
x86/kvm/mmu: make space for source data caching in struct kvm_mmu
x86/kvm/nVMX: introduce scache for kvm_init_shadow_ept_mmu
x86/kvm/mmu: check if tdp/shadow MMU reconfiguration is needed
x86/kvm/mmu: check if MMU reconfiguration is needed in
init_kvm_nested_mmu()

arch/x86/include/asm/kvm_host.h | 42 ++++-
arch/x86/kvm/mmu.c | 330 +++++++++++++++++++++++-----------------
arch/x86/kvm/mmu.h | 8 +-
arch/x86/kvm/mmu_audit.c | 12 +-
arch/x86/kvm/paging_tmpl.h | 15 +-
arch/x86/kvm/svm.c | 14 +-
arch/x86/kvm/vmx.c | 46 +++---
arch/x86/kvm/x86.c | 22 +--
8 files changed, 295 insertions(+), 194 deletions(-)

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2.14.4