KVM: nVMX: Dynamically compute max VMCS index for vmcs12

Calculate the max VMCS index for vmcs12 by walking the array to find the
actual max index.  Hardcoding the index is prone to bitrot, and the
calculation is only done on KVM bringup (albeit on every CPU, but there
aren't _that_ many null entries in the array).

Fixes: 3c0f99366e34 ("KVM: nVMX: Add a TSC multiplier field in VMCS12")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210618214658.2700765-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 35 insertions, 2 deletions

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index b531e08a095b..183fd9d62fc5 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -6375,6 +6375,40 @@ void nested_vmx_set_vmcs_shadowing_bitmap(void)
 }
 
 /*
+ * Indexing into the vmcs12 uses the VMCS encoding rotated left by 6.  Undo
+ * that madness to get the encoding for comparison.
+ */
+#define VMCS12_IDX_TO_ENC(idx) ((u16)(((u16)(idx) >> 6) | ((u16)(idx) << 10)))
+
+static u64 nested_vmx_calc_vmcs_enum_msr(void)
+{
+	/*
+	 * Note these are the so called "index" of the VMCS field encoding, not
+	 * the index into vmcs12.
+	 */
+	unsigned int max_idx, idx;
+	int i;
+
+	/*
+	 * For better or worse, KVM allows VMREAD/VMWRITE to all fields in
+	 * vmcs12, regardless of whether or not the associated feature is
+	 * exposed to L1.  Simply find the field with the highest index.
+	 */
+	max_idx = 0;
+	for (i = 0; i < nr_vmcs12_fields; i++) {
+		/* The vmcs12 table is very, very sparsely populated. */
+		if (!vmcs_field_to_offset_table[i])
+			continue;
+
+		idx = vmcs_field_index(VMCS12_IDX_TO_ENC(i));
+		if (idx > max_idx)
+			max_idx = idx;
+	}
+
+	return (u64)max_idx << VMCS_FIELD_INDEX_SHIFT;
+}
+
+/*
  * nested_vmx_setup_ctls_msrs() sets up variables containing the values to be
  * returned for the various VMX controls MSRs when nested VMX is enabled.
  * The same values should also be used to verify that vmcs12 control fields are
@@ -6619,8 +6653,7 @@ void nested_vmx_setup_ctls_msrs(struct nested_vmx_msrs *msrs, u32 ept_caps)
 	rdmsrl(MSR_IA32_VMX_CR0_FIXED1, msrs->cr0_fixed1);
 	rdmsrl(MSR_IA32_VMX_CR4_FIXED1, msrs->cr4_fixed1);
 
-	/* highest index: VMX_PREEMPTION_TIMER_VALUE */
-	msrs->vmcs_enum = VMCS12_MAX_FIELD_INDEX << 1;
+	msrs->vmcs_enum = nested_vmx_calc_vmcs_enum_msr();
 }
 
 void nested_vmx_hardware_unsetup(void)