[PATCH 00/23] KVM: MMU: MMU role refactoring

Paolo Bonzini
2022-02-04T11:57:25Z

The TDP MMU has a performance regression compared to the legacy
MMU when CR0 changes often. This was reported for the grsecurity
kernel, which uses CR0.WP to implement kernel W^X. In that case,
each change to CR0.WP unloads the MMU and causes a lot of unnecessary
work. When running nested, this can even cause the L1 to hardly
make progress, as the L0 hypervisor it is overwhelmed by the amount
of MMU work that is needed.

The root cause of the issue is that the "MMU role" in KVM is a mess
that mixes the CPU setup (CR0/CR4/EFER, SMM, guest mode, etc.)
and the shadow page table format. Whenever something is different
between the MMU and the CPU, it is stored as an extra field in struct
kvm_mmu---and for extra bonus complication, sometimes the same thing
is stored in both the role and an extra field.

So, this is the "no functional change intended" part of the changes
required to fix the performance regression. It separates neatly
the shadow page table format ("MMU role") from the guest page table
format ("CPU role"), and removes the duplicate fields. The next
step then is to avoid unloading the MMU as long as the MMU role
stays the same.

Please review!

Paolo

Paolo Bonzini (23):
KVM: MMU: pass uses_nx directly to reset_shadow_zero_bits_mask
KVM: MMU: nested EPT cannot be used in SMM
KVM: MMU: remove valid from extended role
KVM: MMU: constify uses of struct kvm_mmu_role_regs
KVM: MMU: pull computation of kvm_mmu_role_regs to kvm_init_mmu
KVM: MMU: load new PGD once nested two-dimensional paging is
initialized
KVM: MMU: remove kvm_mmu_calc_root_page_role
KVM: MMU: rephrase unclear comment
KVM: MMU: remove "bool base_only" arguments
KVM: MMU: split cpu_role from mmu_role
KVM: MMU: do not recompute root level from kvm_mmu_role_regs
KVM: MMU: remove ept_ad field
KVM: MMU: remove kvm_calc_shadow_root_page_role_common
KVM: MMU: cleanup computation of MMU roles for two-dimensional paging
KVM: MMU: cleanup computation of MMU roles for shadow paging
KVM: MMU: remove extended bits from mmu_role
KVM: MMU: remove redundant bits from extended role
KVM: MMU: fetch shadow EFER.NX from MMU role
KVM: MMU: simplify and/or inline computation of shadow MMU roles
KVM: MMU: pull CPU role computation to kvm_init_mmu
KVM: MMU: store shadow_root_level into mmu_role
KVM: MMU: use cpu_role for root_level
KVM: MMU: replace direct_map with mmu_role.direct

arch/x86/include/asm/kvm_host.h | 13 +-
arch/x86/kvm/mmu.h | 2 +-
arch/x86/kvm/mmu/mmu.c | 408 ++++++++++++--------------------
arch/x86/kvm/mmu/mmu_audit.c | 6 +-
arch/x86/kvm/mmu/paging_tmpl.h | 12 +-
arch/86/kvm/mmu/tdp_mmu.c | 4 +-
arch/x86/kvm/svm/svm.c | 2 +-
arch/x86/kvm/vmx/vmx.c | 2 +-
arch/x86/kvm/x86.c | 12 +-
10 files changed, 178 insertions(+), 284 deletions(-)

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2.31.1