[RFC,00/13] Optimize nSVM TLB flushes

Message

Yosry Ahmed Feb. 5, 2025, 6:23 p.m. UTC

Currently KVM does a TLB flush and an MMU sync on every nested
transition (L1 <-> L2), because it uses the same ASID to run both L1 and
L2.

This series addresses that by giving a separate ASID to L2, adding the
necessary TLB management for it, and properly virtualizing TLB flushes
for L1.

Patch 1 introduces a separate ASID for L2, althoug not properly handled
yet, so it keeps the unconditional flushes.

Patches 2 to 6 are some refactoring and groundwork.

Patches 7 to 12 add the actual TLB management for nSVM, some of which
are items on the TODO list in nested_svm_transition_tlb_flush().

Patch 13 finally stops the unconditional flushes on every nested
transition.

I tested this by booting an L2 and running some basic workloads,
including a CPUID microbenchmark to measure the performance improvement
(numbers in the last patch). I sent the RFC to get feedback on the
general approach, and meanwhile I will try to run more tests that could
exercise TLB flushing.

Yosry Ahmed (13):
  KVM: nSVM: Track the ASID per-VMCB
  KVM: nSVM: Rework svm_flush_tlb_asid() to operate on a given VMCB
  KVM: nSVM: Split nested_svm_transition_tlb_flush() into entry/exit fns
  KVM: SVM: Introduce helpers for updating TLB_CONTROL
  KVM: x86/mmu: rename __kvm_mmu_invalidate_addr()
  KVM: x86/mmu: Allow skipping the gva flush in
    kvm_mmu_invalidate_addr()
  KVM: nSVM: Handle INVLPGA interception correctly
  KVM: nSVM: Flush both L1 and L2 ASIDs on KVM_REQ_TLB_FLUSH
  KVM: nSVM: Handle nested TLB flush requests through TLB_CONTROL
  KVM: nSVM: Flush the TLB if L1 changes L2's ASID
  KVM: nSVM: Do not reset TLB_CONTROL in VMCB02 on nested entry
  KVM: nSVM: Service local TLB flushes before nested transitions
  KVM: nSVM: Stop bombing the TLB on nested transitions

 arch/x86/include/asm/kvm_host.h |  2 +
 arch/x86/include/asm/svm.h      |  6 ---
 arch/x86/kvm/mmu/mmu.c          | 22 +++++---
 arch/x86/kvm/svm/nested.c       | 64 +++++++++++++++-------
 arch/x86/kvm/svm/sev.c          |  4 +-
 arch/x86/kvm/svm/svm.c          | 95 ++++++++++++++++++++++++++-------
 arch/x86/kvm/svm/svm.h          | 33 +++++++++++-
 7 files changed, 170 insertions(+), 56 deletions(-)