linux-kernel - [PATCH v2 19/21] KVM: VMX: Update MTF and ICEBP comments to document KVM's subtle behavior

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Message-Id: <20220614204730.3359543-20-seanjc@google.com>
Date:   Tue, 14 Jun 2022 20:47:28 +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 19/21] KVM: VMX: Update MTF and ICEBP comments to document
 KVM's subtle behavior

Document the oddities of ICEBP interception (trap-like #DB is intercepted
as a fault-like exception), and how using VMX's inner "skip" helper
deliberately bypasses the pending MTF and single-step #DB logic.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/vmx.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 3591fdf7ecf9..91b8e171f232 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1578,9 +1578,13 @@ static void vmx_update_emulated_instruction(struct kvm_vcpu *vcpu)
 
 	/*
 	 * Per the SDM, MTF takes priority over debug-trap exceptions besides
-	 * T-bit traps. As instruction emulation is completed (i.e. at the
-	 * instruction boundary), any #DB exception pending delivery must be a
-	 * debug-trap. Record the pending MTF state to be delivered in
+	 * TSS T-bit traps and ICEBP (INT1).  KVM doesn't emulate T-bit traps
+	 * or ICEBP (in the emulator proper), and skipping of ICEBP after an
+	 * intercepted #DB deliberately avoids single-step #DB and MTF updates
+	 * as ICEBP is higher priority than both.  As instruction emulation is
+	 * completed at this point (i.e. KVM is at the instruction boundary),
+	 * any #DB exception pending delivery must be a debug-trap of lower
+	 * priority than MTF.  Record the pending MTF state to be delivered in
 	 * vmx_check_nested_events().
 	 */
 	if (nested_cpu_has_mtf(vmcs12) &&
@@ -5071,8 +5075,10 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
 			 * instruction.  ICEBP generates a trap-like #DB, but
 			 * despite its interception control being tied to #DB,
 			 * is an instruction intercept, i.e. the VM-Exit occurs
-			 * on the ICEBP itself.  Note, skipping ICEBP also
-			 * clears STI and MOVSS blocking.
+			 * on the ICEBP itself.  Use the inner "skip" helper to
+			 * avoid single-step #DB and MTF updates, as ICEBP is
+			 * higher priority.  Note, skipping ICEBP still clears
+			 * STI and MOVSS blocking.
 			 *
 			 * For all other #DBs, set vmcs.PENDING_DBG_EXCEPTIONS.BS
 			 * if single-step is enabled in RFLAGS and STI or MOVSS
-- 
2.36.1.476.g0c4daa206d-goog