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Message-Id: <20220614204730.3359543-5-seanjc@google.com>
Date: Tue, 14 Jun 2022 20:47:13 +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 04/21] KVM: nVMX: Treat General Detect #DB (DR7.GD=1) as fault-like
Exclude General Detect #DBs, which have fault-like behavior but also have
a non-zero payload (DR6.BD=1), from nVMX's handling of pending debug
traps. Opportunistically rewrite the comment to better document what is
being checked, i.e. "has a non-zero payload" vs. "has a payload", and to
call out the many caveats surrounding #DBs that KVM dodges one way or
another.
Cc: Oliver Upton <oupton@...gle.com>
Cc: Peter Shier <pshier@...gle.com>
Fixes: 684c0422da71 ("KVM: nVMX: Handle pending #DB when injecting INIT VM-exit")
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
arch/x86/kvm/vmx/nested.c | 36 +++++++++++++++++++++++++-----------
1 file changed, 25 insertions(+), 11 deletions(-)
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 33ffc8bcf9cd..61bc80fc4cfa 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -3857,16 +3857,29 @@ static void nested_vmx_inject_exception_vmexit(struct kvm_vcpu *vcpu,
}
/*
- * Returns true if a debug trap is pending delivery.
+ * Returns true if a debug trap is (likely) pending delivery. Infer the class
+ * of a #DB (trap-like vs. fault-like) from the exception payload (to-be-DR6).
+ * Using the payload is flawed because code breakpoints (fault-like) and data
+ * breakpoints (trap-like) set the same bits in DR6 (breakpoint detected), i.e.
+ * this will return false positives if a to-be-injected code breakpoint #DB is
+ * pending (from KVM's perspective, but not "pending" across an instruction
+ * boundary). ICEBP, a.k.a. INT1, is also not reflected here even though it
+ * too is trap-like.
*
- * In KVM, debug traps bear an exception payload. As such, the class of a #DB
- * exception may be inferred from the presence of an exception payload.
+ * KVM "works" despite these flaws as ICEBP isn't currently supported by the
+ * emulator, Monitor Trap Flag is not marked pending on intercepted #DBs (the
+ * #DB has already happened), and MTF isn't marked pending on code breakpoints
+ * from the emulator (because such #DBs are fault-like and thus don't trigger
+ * actions that fire on instruction retire).
*/
-static inline bool vmx_pending_dbg_trap(struct kvm_vcpu *vcpu)
+static inline unsigned long vmx_get_pending_dbg_trap(struct kvm_vcpu *vcpu)
{
- return vcpu->arch.exception.pending &&
- vcpu->arch.exception.nr == DB_VECTOR &&
- vcpu->arch.exception.payload;
+ if (!vcpu->arch.exception.pending ||
+ vcpu->arch.exception.nr != DB_VECTOR)
+ return 0;
+
+ /* General Detect #DBs are always fault-like. */
+ return vcpu->arch.exception.payload & ~DR6_BD;
}
/*
@@ -3878,9 +3891,10 @@ static inline bool vmx_pending_dbg_trap(struct kvm_vcpu *vcpu)
*/
static void nested_vmx_update_pending_dbg(struct kvm_vcpu *vcpu)
{
- if (vmx_pending_dbg_trap(vcpu))
- vmcs_writel(GUEST_PENDING_DBG_EXCEPTIONS,
- vcpu->arch.exception.payload);
+ unsigned long pending_dbg = vmx_get_pending_dbg_trap(vcpu);
+
+ if (pending_dbg)
+ vmcs_writel(GUEST_PENDING_DBG_EXCEPTIONS, pending_dbg);
}
static bool nested_vmx_preemption_timer_pending(struct kvm_vcpu *vcpu)
@@ -3937,7 +3951,7 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
* while delivering the pending exception.
*/
- if (vcpu->arch.exception.pending && !vmx_pending_dbg_trap(vcpu)) {
+ if (vcpu->arch.exception.pending && !vmx_get_pending_dbg_trap(vcpu)) {
if (vmx->nested.nested_run_pending)
return -EBUSY;
if (!nested_vmx_check_exception(vcpu, &exit_qual))
--
2.36.1.476.g0c4daa206d-goog
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