FAILED: patch "[PATCH] KVM: x86: Disallow EFER.LME and EFER.LMA if long mode is not" failed to apply to 6.12-stable tree

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x e62392bf39ebfdf60d1d082799397fe1cbf8dfc5
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026090915-drainer-judicial-082f@gregkh' --subject-prefix 'PATCH 6.12.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From e62392bf39ebfdf60d1d082799397fe1cbf8dfc5 Mon Sep 17 00:00:00 2001
From: Yosry Ahmed <[email protected]>
Date: Mon, 13 Jul 2026 18:10:17 +0000
Subject: [PATCH] KVM: x86: Disallow EFER.LME and EFER.LMA if long mode is not
supported

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.

index c230b18d87e3..67481429ad6b 100644
@@ -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
 

index a4216b858194..0845551c6cab 100644
@@ -6910,6 +6910,9 @@ 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);
 

From: Yosry Ahmed <[email protected]>

[ 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.

---
 arch/x86/kvm/x86.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

index 9e7c9e3edb5b4..5bf719a6a6b9c 100644
@@ -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