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

Sean Christopherson
2025-11-18T22:23:35Z

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@linux.intel.com>
Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: Jon Kohler <jon@nutanix.com>
Signed-off-by: Sean Christopherson <seanjc@google.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