KVM: nVMX: Unconditionally validate CR3 during nested transitions

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Unconditionally check the validity of the incoming CR3 during nested
VM-Enter/VM-Exit to avoid invoking kvm_read_cr3() in the common case
where the guest isn't using PAE paging.  If vmcs.GUEST_CR3 hasn't yet
been cached (common case), kvm_read_cr3() will trigger a VMREAD.  The
VMREAD (~30 cycles) alone is likely slower than nested_cr3_valid()
(~5 cycles if vcpu->arch.maxphyaddr gets a cache hit), and the poor
exchange only gets worse when retpolines are enabled as the call to
kvm_x86_ops.cache_reg() will incur a retpoline (60+ cycles).

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200502043234.12481-3-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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Sean Christopherson 2020-05-01 21:32:26 -07:00 • committed by Paolo Bonzini

parent 56ba77a459

commit 0cc69204e7

1 changed files with 13 additions and 14 deletions

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arch/x86/kvm/vmx/nested.c

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