linux-kernel - Re: [PATCH 2/5] KVM: TDX: Exit with MEMORY_FAULT on unexpected pending S-EPT Violation

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Message-ID: <aIlRONVnWJiVbcCv@google.com>
Date: Tue, 29 Jul 2025 15:54:48 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Rick P Edgecombe <rick.p.edgecombe@...el.com>
Cc: "maz@...nel.org" <maz@...nel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"oliver.upton@...ux.dev" <oliver.upton@...ux.dev>, Vishal Annapurve <vannapurve@...gle.com>, 
	Xiaoyao Li <xiaoyao.li@...el.com>, "kvmarm@...ts.linux.dev" <kvmarm@...ts.linux.dev>, 
	Adrian Hunter <adrian.hunter@...el.com>, 
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "nik.borisov@...e.com" <nik.borisov@...e.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] KVM: TDX: Exit with MEMORY_FAULT on unexpected
 pending S-EPT Violation

On Tue, Jul 29, 2025, Rick P Edgecombe wrote:
> On Tue, 2025-07-29 at 12:33 -0700, Sean Christopherson wrote:
> > diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> > index 3e0d4edee849..c2ef03f39c32 100644
> > --- a/arch/x86/kvm/vmx/tdx.c
> > +++ b/arch/x86/kvm/vmx/tdx.c
> > @@ -1937,10 +1937,8 @@ static int tdx_handle_ept_violation(struct kvm_vcpu *vcpu)
> >  
> >  	if (vt_is_tdx_private_gpa(vcpu->kvm, gpa)) {
> >  		if (tdx_is_sept_violation_unexpected_pending(vcpu)) {
> > -			pr_warn("Guest access before accepting 0x%llx on vCPU %d\n",
> > -				gpa, vcpu->vcpu_id);
> > -			kvm_vm_dead(vcpu->kvm);
> > -			return -EIO;
> > +			kvm_prepare_memory_fault_exit(vcpu, gpa, 0, true, false, true);
> > +			return -EFAULT;
> >  		}
> >  		/*
> >  		 * Always treat SEPT violations as write faults.  Ignore the
> 
> The vm_dead was added because mirror EPT will KVM_BUG_ON() if there is an
> attempt to set the mirror EPT entry when it is already present. And the
> unaccepted memory access will trigger an EPT violation for a mirror PTE that is
> already set. I think this is a better solution irrespective of the vm_dead
> changes.

In that case, this change will expose KVM to the KVM_BUG_ON(), because nothing
prevents userspace from re-running the vCPU.  Which KVM_BUG_ON() exactly gets
hit?