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

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Message-ID: <aRtDpQ9DTonYw9Bi@google.com>
Date: Mon, 17 Nov 2025 07:47:49 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Tony Lindgren <tony.lindgren@...ux.intel.com>
Cc: Rick P Edgecombe <rick.p.edgecombe@...el.com>, Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Jon Kohler <jon@...anix.com>
Subject: Re: [PATCH 2/4] KVM: VMX: Handle #MCs on VM-Enter/TD-Enter outside of
 the fastpath

On Mon, Nov 17, 2025, Tony Lindgren wrote:
> Hi,
> 
> On Thu, Oct 30, 2025 at 03:42:44PM -0700, Sean Christopherson wrote:
> > --- a/arch/x86/kvm/vmx/main.c
> > +++ b/arch/x86/kvm/vmx/main.c
> > @@ -608,6 +608,17 @@ static void vt_load_mmu_pgd(struct kvm_vcpu *vcpu, hpa_t root_hpa,
> >  	vmx_load_mmu_pgd(vcpu, root_hpa, pgd_level);
> >  }
> >  
> > +static void vt_handle_exit_irqoff(struct kvm_vcpu *vcpu)
> > +{
> > +	if (unlikely((u16)vmx_get_exit_reason(vcpu).basic == EXIT_REASON_MCE_DURING_VMENTRY))
> > +		kvm_machine_check();
> > +
> > +	if (is_td_vcpu(vcpu))
> > +		return;
> > +
> > +	return vmx_handle_exit_irqoff(vcpu);
> > +}
> 
> I bisected kvm-x86/next down to this change for a TDX guest not booting
> and host producing errors like:
> 
> watchdog: CPU118: Watchdog detected hard LOCKUP on cpu 118
> 
> Dropping the is_td_vcpu(vcpu) check above fixes the issue. Earlier the
> call for vmx_handle_exit_irqoff() was unconditional.

Ugh, once you see it, it's obvious.  Sorry :-(

I'll drop the entire series and send a v2.  There's only one other patch that I
already sent the "thank you" for, so I think it's worth unwinding to avoid
breaking bisection for TDX (and because the diff can be very different).

Lightly tested, but I think this patch can instead be:

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 163f854a39f2..6d41d2fc8043 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1063,9 +1063,6 @@ 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 d98107a7bdaa..d1117da5463f 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7035,10 +7035,19 @@ 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;
+       }
 }
 
 /*
@@ -7501,9 +7510,6 @@ 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))