FAILED: patch "[PATCH] KVM: nVMX: Unconditionally clear nested.pi_pending on nested" failed to apply to 4.9-stable tree

The patch below does not apply to the 4.9-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,

greg k-h

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

From f7782bb8d818d8f47c26b22079db10599922787a Mon Sep 17 00:00:00 2001
From: Sean Christopherson <[email protected]>
Date: Tue, 10 Aug 2021 07:45:26 -0700
Subject: [PATCH] KVM: nVMX: Unconditionally clear nested.pi_pending on nested
VM-Enter

Clear nested.pi_pending on nested VM-Enter even if L2 will run without posted interrupts enabled. If nested.pi_pending is left set from a previous L2, vmx_complete_nested_posted_interrupt() will pick up the stale flag and exit to userspace with an "internal emulation error" due the new L2 not having a valid nested.pi_desc.

Arguably, vmx_complete_nested_posted_interrupt() should first check for posted interrupts being enabled, but it's also completely reasonable that KVM wouldn't screw up a fundamental flag. Not to mention that the mere existence of nested.pi_pending is a long-standing bug as KVM shouldn't move the posted interrupt out of the IRR until it's actually processed, e.g. KVM effectively drops an interrupt when it performs a nested VM-Exit with a "pending" posted interrupt. Fixing the mess is a future problem.

Prior to vmx_complete_nested_posted_interrupt() interpreting a null PI descriptor as an error, this was a benign bug as the null PI descriptor effectively served as a check on PI not being enabled. Even then, the new flow did not become problematic until KVM started checking the result of kvm_check_nested_events().

Message-Id: <[email protected]>

index 264a9f4c9179..bc6327950657 100644
@@ -2187,12 +2187,11 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct loaded_vmcs *vmcs0
 			 ~PIN_BASED_VMX_PREEMPTION_TIMER);
 
 	/* Posted interrupts setting is only taken from vmcs12.  */
-	if (nested_cpu_has_posted_intr(vmcs12)) {
+	vmx->nested.pi_pending = false;
+	if (nested_cpu_has_posted_intr(vmcs12))
 		vmx->nested.posted_intr_nv = vmcs12->posted_intr_nv;
-		vmx->nested.pi_pending = false;
-	} else {
+	else
 		exec_control &= ~PIN_BASED_POSTED_INTR;
-	}
 	pin_controls_set(vmx, exec_control);
 
 	/*