FAILED: Patch "KVM: x86: Ignore -EBUSY when checking nested events from vcpu_block()" failed to apply to 6.6-stable tree

The patch below does not apply to the 6.6-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]>.

Thanks,
Sasha

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

From ead63640d4e72e6f6d464f4e31f7fecb79af8869 Mon Sep 17 00:00:00 2001
From: Sean Christopherson <[email protected]>
Date: Thu, 8 Jan 2026 19:06:57 -0800
Subject: [PATCH] KVM: x86: Ignore -EBUSY when checking nested events from
vcpu_block()

Ignore -EBUSY when checking nested events after exiting a blocking state while L2 is active, as exiting to userspace will generate a spurious userspace exit, usually with KVM_EXIT_UNKNOWN, and likely lead to the VM's demise. Continuing with the wakeup isn't perfect either, as *something* has gone sideways if a vCPU is awakened in L2 with an injected event (or worse, a nested run pending), but continuing on gives the VM a decent chance of surviving without any major side effects.

As explained in the Fixes commits, it _should_ be impossible for a vCPU to be put into a blocking state with an already-injected event (exception, IRQ, or NMI). Unfortunately, userspace can stuff MP_STATE and/or injected events, and thus put the vCPU into what should be an impossible state.

Don't bother trying to preserve the WARN, e.g. with an anti-syzkaller Kconfig, as WARNs can (hopefully) be added in paths where _KVM_ would be violating x86 architecture, e.g. by WARNing if KVM attempts to inject an exception or interrupt while the vCPU isn't running.

---
 arch/x86/kvm/x86.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

index e4418409b468d..fe9d324da72ab 100644
@@ -11597,8 +11597,7 @@ static inline int vcpu_block(struct kvm_vcpu *vcpu)
 	if (is_guest_mode(vcpu)) {
 		int r = kvm_check_nested_events(vcpu);
 
-		WARN_ON_ONCE(r == -EBUSY);
-		if (r < 0)
+		if (r < 0 && r != -EBUSY)
 			return 0;
 	}
 

2.51.0