[PATCH v3 2/5] KVM: x86: Disallow EFER.LME and EFER.LMA if long mode is not supported

[Yosry Ahmed]

Remove EFER.LME and EFER.LMA from EFER reserved bits only if long mode
is actually supported. KVM does check long-mode support before allowing
the bits for guest writes and userspace writes through KVM_SET_SREGS*
(in __kvm_valid_efer()), but userspace writes through KVM_SET_MSRS only
check reserved bits.

In practice, this doesn't really matter. The true motiviation is getting
rid of the #ifdeffery when initializing efer_reserved_bits.

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Yosry Ahmed <yosry@xxxxxxxxxx>
---
 arch/x86/kvm/msrs.c | 10 +---------
 arch/x86/kvm/x86.c  |  3 +++
 2 files changed, 4 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/msrs.c b/arch/x86/kvm/msrs.c
index c230b18d87e38..67481429ad6b8 100644
--- a/arch/x86/kvm/msrs.c
+++ b/arch/x86/kvm/msrs.c
@@ -19,16 +19,8 @@ bool __read_mostly report_ignored_msrs = true;
 module_param(report_ignored_msrs, bool, 0644);
 EXPORT_SYMBOL_FOR_KVM_INTERNAL(report_ignored_msrs);
 
-/* EFER defaults:
- * - enable syscall per default because its emulated by KVM
- * - enable LME and LMA per default on 64 bit KVM
- */
-#ifdef CONFIG_X86_64
-static
-u64 __read_mostly efer_reserved_bits = ~((u64)(EFER_SCE | EFER_LME | EFER_LMA));
-#else
+/* Enable syscall by default because its emulated by KVM */
 static u64 __read_mostly efer_reserved_bits = ~((u64)EFER_SCE);
-#endif
 
 #define MAX_IO_MSRS 256
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ca01a2f2ec466..f68424a985cda 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6913,6 +6913,9 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_setup_xss_caps);
 
 static void kvm_setup_efer_caps(void)
 {
+	if (kvm_cpu_cap_has(X86_FEATURE_LM))
+		kvm_enable_efer_bits(EFER_LME | EFER_LMA);
+
 	if (kvm_cpu_cap_has(X86_FEATURE_NX))
 		kvm_enable_efer_bits(EFER_NX);
 
-- 
2.55.0.141.g00534a21ce-goog