FAILED: patch "[PATCH] KVM: VMX: Formally define KVM's "0xdead" exit reason used for" failed to apply to 6.18-stable tree

The patch below does not apply to the 6.18-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-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x d85f7dc8cf3c91a131946dae4c3aa8d9c97322d7
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026110438-shrink-panic-2119@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

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

From d85f7dc8cf3c91a131946dae4c3aa8d9c97322d7 Mon Sep 17 00:00:00 2001
From: Sean Christopherson <[email protected]>
Date: Mon, 21 Sep 2026 16:56:55 -0700
Subject: [PATCH] KVM: VMX: Formally define KVM's "0xdead" exit reason used for
failed VM-Enter

Add EXIT_REASON_UNDEFINED to track KVM's magic "0xdead" value that's used to stuff the exit reason on a failed VM-Enter so that the magic can be reused by TDX, and to make it more obvious that it's an arbitrary, KVM- defined value (though the whole "dead" thing makes it pretty darn obvious).

Opportunistically add a compile-time assert to ensure the KVM-defined exit reason never collides with a "real" exit reason.

No functional change intended.

index 3f1b3096ff04..e49b836c7313 100644
@@ -20,6 +20,14 @@
 #include <asm/trapnr.h>
 #include <asm/vmxfeatures.h>
 
+/*
+ * UNDEFINED is a KVM-defined value used to indicate that the basic exit reason
+ * is undefined/invalid, e.g. on VM-Fail or if TDX never attempted VM-Enter.
+ * The value itself is arbitrary, it just needs to not collide with a real exit
+ * reason.
+ */
+#define EXIT_REASON_UNDEFINED			0xdead
+
 struct vmcs_hdr {
 	u32 revision_id:31;
 	u32 shadow_vmcs:1;

index 612ab07d4100..c897dbf0e954 100644
@@ -6388,6 +6388,8 @@ static int (*kvm_vmx_exit_handlers[])(struct kvm_vcpu *vcpu) = {
 static const int kvm_vmx_max_exit_handlers =
 	ARRAY_SIZE(kvm_vmx_exit_handlers);
 
+static_assert(EXIT_REASON_UNDEFINED >= ARRAY_SIZE(kvm_vmx_exit_handlers));
+
 void vmx_get_exit_info(struct kvm_vcpu *vcpu, u32 *reason,
 		       u64 *info1, u64 *info2, u32 *intr_info, u32 *error_code)
 {

@@ -7474,7 +7476,7 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
 	vmx_enable_fb_clear(vmx);
 
 	if (unlikely(vmx->fail)) {
-		vmx->vt.exit_reason.full = 0xdead;
+		vmx->vt.exit_reason.full = EXIT_REASON_UNDEFINED;
 		goto out;
 	}
 

From: Sean Christopherson <[email protected]>

[ Upstream commit d85f7dc8cf3c91a131946dae4c3aa8d9c97322d7 ]

Add EXIT_REASON_UNDEFINED to track KVM's magic "0xdead" value that's used to stuff the exit reason on a failed VM-Enter so that the magic can be reused by TDX, and to make it more obvious that it's an arbitrary, KVM- defined value (though the whole "dead" thing makes it pretty darn obvious).

Opportunistically add a compile-time assert to ensure the KVM-defined exit reason never collides with a "real" exit reason.

No functional change intended.

---
 arch/x86/include/asm/vmx.h | 8 ++++++++
 arch/x86/kvm/vmx/vmx.c     | 4 +++-
 2 files changed, 11 insertions(+), 1 deletion(-)

index c85c50019523..a9daddd84ad6 100644
@@ -20,6 +20,14 @@
 #include <asm/trapnr.h>
 #include <asm/vmxfeatures.h>
 
+/*
+ * UNDEFINED is a KVM-defined value used to indicate that the basic exit reason
+ * is undefined/invalid, e.g. on VM-Fail or if TDX never attempted VM-Enter.
+ * The value itself is arbitrary, it just needs to not collide with a real exit
+ * reason.
+ */
+#define EXIT_REASON_UNDEFINED			0xdead
+
 #define VMCS_CONTROL_BIT(x)	BIT(VMX_FEATURE_##x & 0x1f)
 
 /*

index 5bae3d5ce6ee..54a78d5e9fe7 100644
@@ -6173,6 +6173,8 @@ static int (*kvm_vmx_exit_handlers[])(struct kvm_vcpu *vcpu) = {
 static const int kvm_vmx_max_exit_handlers =
 	ARRAY_SIZE(kvm_vmx_exit_handlers);
 
+static_assert(EXIT_REASON_UNDEFINED >= ARRAY_SIZE(kvm_vmx_exit_handlers));
+
 void vmx_get_exit_info(struct kvm_vcpu *vcpu, u32 *reason,
 		       u64 *info1, u64 *info2, u32 *intr_info, u32 *error_code)
 {

@@ -7354,7 +7356,7 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
 	vmx_enable_fb_clear(vmx);
 
 	if (unlikely(vmx->fail)) {
-		vmx->vt.exit_reason.full = 0xdead;
+		vmx->vt.exit_reason.full = EXIT_REASON_UNDEFINED;
 		goto out;
 	}
 

2.53.0

In the future, please provide a quick explanation of the conflict resolution(s), it helps me assess the safety/correctness of the backport, and provides a paper trail to help with debug in case the backport introduces a bug.

For this, there was a superficial conflict due to lack of upstream commit 3c75e6a5da3c0 ("KVM: VMX: Move architectural "vmcs" and "vmcs_hdr" structures to public vmx.h").

Queued for 6.18 and 6.12, thanks.

Thanks,
Sasha