[PATCH 15/15] KVM: nVMX: Always write vmcs.GUEST_CR3 during nested VM-Exit

Lai Jiangshan
2021-11-08T12:45:55Z

From: Lai Jiangshan <laijs@linux.alibaba.com>

For VM-Enter, vmcs.GUEST_CR3 and vcpu->arch.cr3 are synced and it is
better to mark VCPU_EXREG_CR3 available rather than dirty to reduce a
redundant vmwrite(GUEST_CR3) in vmx_load_mmu_pgd().

But nested_vmx_load_cr3() is also served for VM-Exit which doesn't
set vmcs.GUEST_CR3.

This patch moves writing to vmcs.GUEST_CR3 into nested_vmx_load_cr3()
for both nested VM-Eneter/Exit and use kvm_register_mark_available().

This patch doesn't cause any extra writing to vmcs.GUEST_CR3 and if
userspace is modifying CR3 with KVM_SET_SREGS later, the dirty info
for VCPU_EXREG_CR3 would be set for next writing to vmcs.GUEST_CR3
and no update will be lost.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
arch/x86/kvm/vmx/nested.c | 32 +++++++++++++++++++++-----------
1 file changed, 21 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index ee5a68c2ea3a..4ddd4b1b0503 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -1133,8 +1133,28 @@ static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3,
if (!nested_ept)
kvm_mmu_new_pgd(vcpu, cr3);

+ /*
+ * Immediately write vmcs.GUEST_CR3 when changing vcpu->arch.cr3.
+ *
+ * VCPU_EXREG_CR3 is marked available rather than dirty because
+ * vcpu->arch.cr3 and vmcs.GUEST_CR3 are synced when enable_ept and
+ * vmcs.GUEST_CR3 is irrelevant to vcpu->arch.cr3 when !enable_ept.
+ *
+ * For VM-Enter case, it will be propagated to vmcs12 on nested
+ * VM-Exit, which can occur without actually running L2 and thus
+ * without hitting vmx_load_mmu_pgd(), e.g. if L1 is entering L2 with
+ * vmcs12.GUEST_ACTIVITYSTATE=HLT, in which case KVM will intercept
+ * the transition to HLT instead of running L2.
+ *
+ * For VM-Exit case, it is likely that vmcs.GUEST_CR3 == cr3 here, but
+ * L1 may set HOST_CR3 to a value other than its CR3 before VM-Entry,
+ * so we just update it unconditionally.
+ */
+ if (enable_ept)
+ vmcs_writel(GUEST_CR3, cr3);
+
vcpu->arch.cr3 = cr3;
- kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
+ kvm_register_mark_available(vcpu, VCPU_EXREG_CR3);

/* Re-initialize the MMU, e.g. to pick up CR4 MMU role changes. */
kvm_init_mmu(vcpu);
@@ -2600,16 +2620,6 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
from_vmentry, entry_failure_code))
return -EINVAL;

- /*
- * Immediately write vmcs02.GUEST_CR3. It will be propagated to vmcs12
- * on nested VM-Exit, which can occur without actually running L2 and
- * thus without hitting vmx_load_mmu_pgd(), e.g. if L1 is entering L2 with
- * vmcs12.GUEST_ACTIVITYSTATE=HLT, in which case KVM will intercept the
- * transition to HLT instead of running L2.
- */
- if (enable_ept)
- vmcs_writel(GUEST_CR3, vmcs12->guest_cr3);
-
/* Late preparation of GUEST_PDPTRs now that EFER and CRs are set. */
if (load_guest_pdptrs_vmcs12 && nested_cpu_has_ept(vmcs12) &&
is_pae_paging(vcpu)) {
--
2.19.1.6.gb485710b