[PATCH 6.12.y] KVM: x86: Disallow EFER.LME and EFER.LMA if long mode is not supported

[Sasha Levin <sashal@kernel.org>]

From: Yosry Ahmed <yosry@kernel.org>

[ Upstream commit e62392bf39ebfdf60d1d082799397fe1cbf8dfc5 ]

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@vger.kernel.org
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260713181020.2735367-3-yosry@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
[ adapted EFER changes to x86.c, placing the capability check in kvm_x86_vendor_init() because kvm_setup_efer_caps() is absent. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/x86/kvm/x86.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9e7c9e3edb5b4..5bf719a6a6b9c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -108,16 +108,8 @@ EXPORT_SYMBOL_GPL(kvm_host);
 #define emul_to_vcpu(ctxt) \
 	((struct kvm_vcpu *)(ctxt)->vcpu)
 
-/* 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
 
 static u64 __read_mostly cr4_reserved_bits = CR4_RESERVED_BITS;
 
@@ -9822,6 +9814,9 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
 	if (r != 0)
 		goto out_mmu_exit;
 
+	if (kvm_cpu_cap_has(X86_FEATURE_LM))
+		kvm_enable_efer_bits(EFER_LME | EFER_LMA);
+
 	kvm_ops_update(ops);
 
 	for_each_online_cpu(cpu) {
-- 
2.53.0