[PATCH 4/4] KVM: nVMX: Map enlightened VMCS upon restore when possible

Vitaly Kuznetsov
2021-05-03T15:09:19Z

It now looks like a bad idea to not restore eVMCS mapping directly from
vmx_set_nested_state(). The restoration path now depends on whether KVM
will continue executing L2 (vmx_get_nested_state_pages()) or will have to
exit to L1 (nested_vmx_vmexit()), this complicates error propagation and
diverges too much from the 'native' path when 'nested.current_vmptr' is
set directly from vmx_get_nested_state_pages().

The existing solution postponing eVMCS mapping also seems to be fragile.
In multiple places the code checks whether 'vmx->nested.hv_evmcs' is not
NULL to distinguish between eVMCS and non-eVMCS cases. All these checks
are 'incomplete' as we have a weird 'eVMCS is in use but not yet mapped'
state.

Also, in case vmx_get_nested_state() is called right after
vmx_set_nested_state() without executing the guest first, the resulting
state is going to be incorrect as 'KVM_STATE_NESTED_EVMCS' flag will be
missing.

Fix all these issues by making eVMCS restoration path closer to its
'native' sibling by putting eVMCS GPA to 'struct kvm_vmx_nested_state_hdr'.
To avoid ABI incompatibility, do not introduce a new flag and keep the
original eVMCS mapping path through KVM_REQ_GET_NESTED_STATE_PAGES in
place. To distinguish between 'new' and 'old' formats consider eVMCS
GPA == 0 as an unset GPA (thus forcing KVM_REQ_GET_NESTED_STATE_PAGES
path). While technically possible, it seems to be an extremely unlikely
case.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
arch/x86/include/uapi/asm/kvm.h | 2 ++
arch/x86/kvm/vmx/nested.c | 27 +++++++++++++++++++++------
2 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index 0662f644aad9..3845977b739e 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -441,6 +441,8 @@ struct kvm_vmx_nested_state_hdr {

__u32 flags;
__u64 preemption_timer_deadline;
+
+ __u64 evmcs_pa;
};

struct kvm_svm_nested_state_data {
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 37fdc34f7afc..4261cf4755c8 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -6019,6 +6019,7 @@ static int vmx_get_nested_state(struct kvm_vcpu *vcpu,
.hdr.vmx.vmxon_pa = -1ull,
.hdr.vmx.vmcs12_pa = -1ull,
.hdr.vmx.preemption_timer_deadline = 0,
+ .hdr.vmx.evmcs_pa = -1ull,
};
struct kvm_vmx_nested_state_data __user *user_vmx_nested_state =
&user_kvm_nested_state->data.vmx[0];
@@ -6037,8 +6038,10 @@ static int vmx_get_nested_state(struct kvm_vcpu *vcpu,
if (vmx_has_valid_vmcs12(vcpu)) {
kvm_state.size += sizeof(user_vmx_nested_state->vmcs12);

- if (vmx->nested.hv_evmcs)
+ if (vmx->nested.hv_evmcs) {
kvm_state.flags |= KVM_STATE_NESTED_EVMCS;
+ kvm_state.hdr.vmx.evmcs_pa = vmx->nested.hv_evmcs_vmptr;
+ }

if (is_guest_mode(vcpu) &&
nested_cpu_has_shadow_vmcs(vmcs12) &&
@@ -6230,13 +6233,25 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,

set_current_vmptr(vmx, kvm_state->hdr.vmx.vmcs12_pa);
} else if (kvm_state->flags & KVM_STATE_NESTED_EVMCS) {
+ u64 evmcs_gpa = kvm_state->hdr.vmx.evmcs_pa;
+
/*
- * nested_vmx_handle_enlightened_vmptrld() cannot be called
- * directly from here as HV_X64_MSR_VP_ASSIST_PAGE may not be
- * restored yet. EVMCS will be mapped from
- * nested_get_vmcs12_pages().
+ * EVMCS GPA == 0 most likely indicates that the migration data is
+ * coming from an older KVM which doesn't support 'evmcs_pa' in
+ * 'struct kvm_vmx_nested_state_hdr'.
*/
- kvm_make_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu);
+ if (evmcs_gpa && (evmcs_gpa != -1ull) &&
+ (__nested_vmx_handle_enlightened_vmptrld(vcpu, evmcs_gpa, false) !=
+ EVMPTRLD_SUCCEEDED)) {
+ return -EINVAL;
+ } else if (!evmcs_gpa) {
+ /*
+ * EVMCS GPA can't be acquired from VP assist page here because
+ * HV_X64_MSR_VP_ASSIST_PAGE may not be restored yet.
+ * EVMCS will be mapped from nested_get_evmcs_page().
+ */
+ kvm_make_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu);
+ }
} else {
return -EINVAL;
}
--
2.30.2