linux-kernel - [PATCH RFC 0/7] x86/kvm/nVMX: optimize MMU switch between L1 and L2

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Date:   Fri, 20 Jul 2018 15:26:19 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Liran Alon <liran.alon@...cle.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH RFC 0/7] x86/kvm/nVMX: optimize MMU switch between L1 and L2

Currently, when we switch from L1 to L2 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())
- Reload MMU root upon guest entry.

When we switch back we do:
- Re-initialize L1 MMU (nested_vmx_load_cr3() -> init_kvm_tdp_mmu())
- Reload MMU root upon guest entry.

This seems to be sub-optimal. Initializing MMU is expensive (thanks to
update_permission_bitmask(), update_pkru_bitmask(),..) and reloading MMU
root doesn't come for free.

Try to approach 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.

RFC part:
- Does this look like a plausible solution?
- SVM nested can probably be optimized in the same way.
- Doesn mmu_update_needed() cover everything?

Vitaly Kuznetsov (7):
  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: get rid of redundant kvm_mmu_setup()
  x86/kvm/nVMX: introduce scache for kvm_init_shadow_ept_mmu
  x86/kvm/nVMX: optimize MMU switch from nested_vmx_load_cr3()

 arch/x86/include/asm/kvm_host.h |  36 ++++-
 arch/x86/kvm/cpuid.c            |   2 +-
 arch/x86/kvm/mmu.c              | 282 ++++++++++++++++++++++++++--------------
 arch/x86/kvm/mmu.h              |   2 +-
 arch/x86/kvm/mmu_audit.c        |  12 +-
 arch/x86/kvm/paging_tmpl.h      |  17 +--
 arch/x86/kvm/svm.c              |  20 +--
 arch/x86/kvm/vmx.c              |  52 +++++---
 arch/x86/kvm/x86.c              |  34 ++---
 9 files changed, 292 insertions(+), 165 deletions(-)

-- 
2.14.4