[PATCH v3 7/9] KVM: nSVM: implement nested_svm_load_cr3() and use it for host->guest switch

Vitaly Kuznetsov
2020-07-09T14:54:48Z

Undesired triple fault gets injected to L1 guest on SVM when L2 is
launched with certain CR3 values. #TF is raised by mmu_check_root()
check in fast_pgd_switch() and the root cause is that when
kvm_set_cr3() is called from nested_prepare_vmcb_save() with NPT
enabled CR3 points to a nGPA so we can't check it with
kvm_is_visible_gfn().

Using generic kvm_set_cr3() when switching to nested guest is not
a great idea as we'll have to distinguish between 'real' CR3s and
'nested' CR3s to e.g. not call kvm_mmu_new_pgd() with nGPA. Following
nVMX implement nested-specific nested_svm_load_cr3() doing the job.

Note: the current implementation is sub-optimal as we always do TLB
flush/MMU sync but this is still an improvement as we at least stop doing
kvm_mmu_reset_context().

Fixes: 7c390d350f8b ("kvm: x86: Add fast CR3 switch code path")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
arch/x86/kvm/mmu/mmu.c | 2 ++
arch/x86/kvm/svm/nested.c | 33 +++++++++++++++++++++++++--------
2 files changed, 27 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 69fa51af8cbf..1c3a231f825b 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4986,6 +4986,8 @@ void kvm_init_shadow_npt_mmu(struct kvm_vcpu *vcpu, u32 cr0, u32 cr4, u32 efer,
union kvm_mmu_role new_role =
kvm_calc_shadow_mmu_root_page_role(vcpu, false);

+ __kvm_mmu_new_pgd(vcpu, nested_cr3, new_role.base, false, false);
+
if (new_role.as_u64 != context->mmu_role.as_u64)
shadow_mmu_init_context(vcpu, context, cr0, cr4, efer, new_role);
}
diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 5f001d2c41d1..5ddf20941cf9 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -323,7 +323,28 @@ static inline bool nested_npt_enabled(struct vcpu_svm *svm)
static int nested_svm_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3,
bool nested_npt)
{
- return kvm_set_cr3(vcpu, cr3);
+ if (cr3 & rsvd_bits(cpuid_maxphyaddr(vcpu), 63))
+ return -EINVAL;
+
+ if (!nested_npt && is_pae_paging(vcpu) &&
+ (cr3 != kvm_read_cr3(vcpu) || pdptrs_changed(vcpu))) {
+ if (!load_pdptrs(vcpu, vcpu->arch.walk_mmu, cr3))
+ return -EINVAL;
+ }
+
+ /*
+ * TODO: optimize unconditional TLB flush/MMU sync here and in
+ * kvm_init_shadow_npt_mmu().
+ */
+ if (!nested_npt)
+ kvm_mmu_new_pgd(vcpu, cr3, false, false);
+
+ vcpu->arch.cr3 = cr3;
+ kvm_register_mark_available(vcpu, VCPU_EXREG_CR3);
+
+ kvm_init_mmu(vcpu, false);
+
+ return 0;
}

static int nested_prepare_vmcb_save(struct vcpu_svm *svm, struct vmcb *nested_vmcb)
@@ -342,6 +363,9 @@ static int nested_prepare_vmcb_save(struct vcpu_svm *svm, struct vmcb *nested_vm
svm_set_cr0(&svm->vcpu, nested_vmcb->save.cr0);
svm_set_cr4(&svm->vcpu, nested_vmcb->save.cr4);

+ if (nested_npt_enabled(svm))
+ nested_svm_init_mmu_context(&svm->vcpu);
+
ret = nested_svm_load_cr3(&svm->vcpu, nested_vmcb->save.cr3,
nested_npt_enabled(svm));

@@ -364,13 +388,6 @@ static int nested_prepare_vmcb_save(struct vcpu_svm *svm, struct vmcb *nested_vm
static void nested_prepare_vmcb_control(struct vcpu_svm *svm)
{
const u32 mask = V_INTR_MASKING_MASK | V_GIF_ENABLE_MASK | V_GIF_MASK;
- if (nested_npt_enabled(svm))
- nested_svm_init_mmu_context(&svm->vcpu);
-
- /* Guest paging mode is active - reset mmu */
- kvm_mmu_reset_context(&svm->vcpu);
-
- svm_flush_tlb(&svm->vcpu);

svm->vmcb->control.tsc_offset = svm->vcpu.arch.tsc_offset =
svm->vcpu.arch.l1_tsc_offset + svm->nested.ctl.tsc_offset;
--
2.25.4