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KVM: nVMX: Unconditionally validate CR3 during nested transitions 

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>
This commit is contained in:
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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27
arch/x86/kvm/vmx/nested.c
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@ -1119,21 +1119,20 @@ static bool nested_vmx_transition_mmu_sync(struct kvm_vcpu *vcpu)
static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3, bool nested_ept,
u32 *entry_failure_code)
{
if (cr3 != kvm_read_cr3(vcpu) || (!nested_ept && pdptrs_changed(vcpu))) {
if (CC(!nested_cr3_valid(vcpu, cr3))) {
*entry_failure_code = ENTRY_FAIL_DEFAULT;
return -EINVAL;
}
if (CC(!nested_cr3_valid(vcpu, cr3))) {
*entry_failure_code = ENTRY_FAIL_DEFAULT;
return -EINVAL;
}
/*
* If PAE paging and EPT are both on, CR3 is not used by the CPU and
* must not be dereferenced.
*/
if (is_pae_paging(vcpu) && !nested_ept) {
if (CC(!load_pdptrs(vcpu, vcpu->arch.walk_mmu, cr3))) {
*entry_failure_code = ENTRY_FAIL_PDPTE;
return -EINVAL;
}
/*
* If PAE paging and EPT are both on, CR3 is not used by the CPU and
* must not be dereferenced.
*/
if (!nested_ept && is_pae_paging(vcpu) &&
(cr3 != kvm_read_cr3(vcpu) || pdptrs_changed(vcpu))) {
if (CC(!load_pdptrs(vcpu, vcpu->arch.walk_mmu, cr3))) {
*entry_failure_code = ENTRY_FAIL_PDPTE;
return -EINVAL;
}
}