[SRU][Focal][PATCH 1/1] KVM: x86: Set KVM_REQ_EVENT if run is canceled with req_immediate_exit set

[Matthew Ruffell]

From: Sean Christopherson <sean.j.christopherson at intel.com>

BugLink: https://bugs.launchpad.net/bugs/1911848

Re-request KVM_REQ_EVENT if vcpu_enter_guest() bails after processing
pending requests and an immediate exit was requested.  This fixes a bug
where a pending event, e.g. VMX preemption timer, is delayed and/or lost
if the exit was deferred due to something other than a higher priority
_injected_ event, e.g. due to a pending nested VM-Enter.  This bug only
affects the !injected case as kvm_x86_ops.cancel_injection() sets
KVM_REQ_EVENT to redo the injection, but that's purely serendipitous
behavior with respect to the deferred event.

Note, emulated preemption timer isn't the only event that can be
affected, it simply happens to be the only event where not re-requesting
KVM_REQ_EVENT is blatantly visible to the guest.

Fixes: f4124500c2c13 ("KVM: nVMX: Fully emulate preemption timer")
Signed-off-by: Sean Christopherson <sean.j.christopherson at intel.com>
Message-Id: <20200423022550.15113-4-sean.j.christopherson at intel.com>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
(backported from commit 8081ad06b68a728e676d3b08e9ab70ce4039747b)
[mruffell: minor context adjustment]
Signed-off-by: Matthew Ruffell <matthew.ruffell at canonical.com>
---
 arch/x86/kvm/x86.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e519e0af028a..dddbeb3ee2c6 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8345,6 +8345,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	return r;
 
 cancel_injection:
+	if (req_immediate_exit)
+		kvm_make_request(KVM_REQ_EVENT, vcpu);
 	kvm_x86_ops->cancel_injection(vcpu);
 	if (unlikely(vcpu->arch.apic_attention))
 		kvm_lapic_sync_from_vapic(vcpu);
-- 
2.27.0