linux-kernel - [PATCH v2 05/21] KVM: nVMX: Prioritize TSS T-flag #DBs over Monitor Trap Flag

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Message-Id: <20220614204730.3359543-6-seanjc@google.com>
Date:   Tue, 14 Jun 2022 20:47:14 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Oliver Upton <oupton@...gle.com>,
        Peter Shier <pshier@...gle.com>
Subject: [PATCH v2 05/21] KVM: nVMX: Prioritize TSS T-flag #DBs over Monitor
 Trap Flag

Service TSS T-flag #DBs prior to pending MTFs, as such #DBs are higher
priority than MTF.  KVM itself doesn't emulate TSS #DBs, and any such
exceptions injected from L1 will be handled by hardware (or morphed to
a fault-like exception if injection fails), but theoretically userspace
could pend a TSS T-flag #DB in conjunction with a pending MTF.

Note, there's no known use case this fixes, it's purely to be technically
correct with respect to Intel's SDM.

Cc: Oliver Upton <oupton@...gle.com>
Cc: Peter Shier <pshier@...gle.com>
Fixes: 5ef8acbdd687 ("KVM: nVMX: Emulate MTF when performing instruction emulation")
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/nested.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 61bc80fc4cfa..e794791a6bdd 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -3943,15 +3943,17 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
 	}
 
 	/*
-	 * Process any exceptions that are not debug traps before MTF.
+	 * Process exceptions that are higher priority than Monitor Trap Flag:
+	 * fault-like exceptions, TSS T flag #DB (not emulated by KVM, but
+	 * could theoretically come in from userspace), and ICEBP (INT1).
 	 *
 	 * Note that only a pending nested run can block a pending exception.
 	 * Otherwise an injected NMI/interrupt should either be
 	 * lost or delivered to the nested hypervisor in the IDT_VECTORING_INFO,
 	 * while delivering the pending exception.
 	 */
-
-	if (vcpu->arch.exception.pending && !vmx_get_pending_dbg_trap(vcpu)) {
+	if (vcpu->arch.exception.pending &&
+	    !(vmx_get_pending_dbg_trap(vcpu) & ~DR6_BT)) {
 		if (vmx->nested.nested_run_pending)
 			return -EBUSY;
 		if (!nested_vmx_check_exception(vcpu, &exit_qual))
-- 
2.36.1.476.g0c4daa206d-goog