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

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Message-ID: <aIm13dDJf4/jGxKv@yzhao56-desk.sh.intel.com>
Date: Wed, 30 Jul 2025 14:04:13 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Xiaoyao Li <xiaoyao.li@...el.com>
CC: Sean Christopherson <seanjc@...gle.com>, Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>, Paolo Bonzini <pbonzini@...hat.com>,
	<linux-arm-kernel@...ts.infradead.org>, <kvmarm@...ts.linux.dev>,
	<kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Adrian Hunter
	<adrian.hunter@...el.com>, Vishal Annapurve <vannapurve@...gle.com>, "Rick
 Edgecombe" <rick.p.edgecombe@...el.com>, Nikolay Borisov
	<nik.borisov@...e.com>
Subject: Re: [PATCH 2/5] KVM: TDX: Exit with MEMORY_FAULT on unexpected
 pending S-EPT Violation

On Wed, Jul 30, 2025 at 10:07:36AM +0800, Xiaoyao Li wrote:
> On 7/30/2025 3:33 AM, Sean Christopherson wrote:
> > Exit to userspace with -EFAULT and a valid MEMORY_FAULT exit if a vCPU
> > hits an unexpected pending S-EPT Violation instead of marking the VM dead.
> > While it's unlikely the VM can continue on, whether or not to terminate
> > the VM is not KVM's decision to make.
> > 
> > Set memory_fault.size to zero to communicate to userspace that reported
> > fault is "bad", and to effectively terminate the VM if userspace blindly
> > treats the exit as a conversion attempt (KVM_SET_MEMORY_ATTRIBUTES will
> > fail with -EINVAL if the size is zero).
> 
> This sets a special contract on size zero.
+1.

Or would it be good to use pr_warn_once() to indicate that the VM termination
reason is "Guest access before accepting" if a new exit type is not specified?

This info is straightforward and helpful in cases when a TD is accidentally
killed.

> I had a patch internally, which introduce a new exit type:
> 
> +               /* KVM_EXIT_GUEST_ERROR */
> +               struct {
> +  #define KVM_GUEST_ERROR_TDX_ACCESS_PENDING_PAGE      0
> +                       __u32 error_type;
> +                       __u32 ndata;
> +                       __u64 data[16];
> +               } guest_error;
> 
> how about it?
> 
> > Opportunistically delete the pr_warn(), which could be abused to spam the
> > kernel log, and is largely useless outside of interact debug as it doesn't
> > specify which VM encountered a failure.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >   arch/x86/kvm/vmx/tdx.c | 6 ++----
> >   1 file changed, 2 insertions(+), 4 deletions(-)
> > 
> > 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
> 
>