linux-kernel - [PATCH v2 2/4] KVM: VMX: Handle #MCs on VM-Enter/TD-Enter outside of the fastpath

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Message-ID: <20251118222328.2265758-3-seanjc@google.com>
Date: Tue, 18 Nov 2025 14:23:26 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	"Kirill A. Shutemov" <kas@...nel.org>
Cc: kvm@...r.kernel.org, x86@...nel.org, linux-coco@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, Rick Edgecombe <rick.p.edgecombe@...el.com>, 
	Jon Kohler <jon@...anix.com>, Tony Lindgren <tony.lindgren@...ux.intel.com>
Subject: [PATCH v2 2/4] KVM: VMX: Handle #MCs on VM-Enter/TD-Enter outside of
 the fastpath

Handle Machine Checks (#MC) that happen on VM-Enter (VMX or TDX) outside
of KVM's fastpath so that as much host state as possible is re-loaded
before invoking the kernel's #MC handler.  The only requirement is that
KVM invokes the #MC handler before enabling IRQs (and even that could
_probably_ be related to handling #MCs before enabling preemption).

Waiting to handle #MCs until "more" host state is loaded hardens KVM
against flaws in the #MC handler, which has historically been quite
brittle. E.g. prior to commit 5567d11c21a1 ("x86/mce: Send #MC singal from
task work"), the #MC code could trigger a schedule() with IRQs and
preemption disabled.  That led to a KVM hack-a-fix in commit 1811d979c716
("x86/kvm: move kvm_load/put_guest_xcr0 into atomic context").

Note, vmx_handle_exit_irqoff() is common to VMX and TDX guests.

Cc: Tony Lindgren <tony.lindgren@...ux.intel.com>
Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc: Jon Kohler <jon@...anix.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/tdx.c |  3 ---
 arch/x86/kvm/vmx/vmx.c | 16 +++++++++++-----
 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index e6105a527372..2d7a4d52ccfb 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1110,9 +1110,6 @@ fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
 	if (unlikely((tdx->vp_enter_ret & TDX_SW_ERROR) == TDX_SW_ERROR))
 		return EXIT_FASTPATH_NONE;
 
-	if (unlikely(vmx_get_exit_reason(vcpu).basic == EXIT_REASON_MCE_DURING_VMENTRY))
-		kvm_machine_check();
-
 	trace_kvm_exit(vcpu, KVM_ISA_VMX);
 
 	if (unlikely(tdx_failed_vmentry(vcpu)))
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index fdcc519348cd..f369c499b2c3 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7062,10 +7062,19 @@ void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu)
 	if (to_vt(vcpu)->emulation_required)
 		return;
 
-	if (vmx_get_exit_reason(vcpu).basic == EXIT_REASON_EXTERNAL_INTERRUPT)
+	switch (vmx_get_exit_reason(vcpu).basic) {
+	case EXIT_REASON_EXTERNAL_INTERRUPT:
 		handle_external_interrupt_irqoff(vcpu, vmx_get_intr_info(vcpu));
-	else if (vmx_get_exit_reason(vcpu).basic == EXIT_REASON_EXCEPTION_NMI)
+		break;
+	case EXIT_REASON_EXCEPTION_NMI:
 		handle_exception_irqoff(vcpu, vmx_get_intr_info(vcpu));
+		break;
+	case EXIT_REASON_MCE_DURING_VMENTRY:
+		kvm_machine_check();
+		break;
+	default:
+		break;
+	}
 }
 
 /*
@@ -7528,9 +7537,6 @@ fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
 	if (unlikely(vmx->fail))
 		return EXIT_FASTPATH_NONE;
 
-	if (unlikely((u16)vmx_get_exit_reason(vcpu).basic == EXIT_REASON_MCE_DURING_VMENTRY))
-		kvm_machine_check();
-
 	trace_kvm_exit(vcpu, KVM_ISA_VMX);
 
 	if (unlikely(vmx_get_exit_reason(vcpu).failed_vmentry))
-- 
2.52.0.rc1.455.g30608eb744-goog