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

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Message-ID: <1b0ea352-c645-461b-9e19-5202791f8e2d@intel.com>
Date: Wed, 30 Jul 2025 10:07:36 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>, Marc Zyngier <maz@...nel.org>,
 Oliver Upton <oliver.upton@...ux.dev>, Paolo Bonzini <pbonzini@...hat.com>
Cc: 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 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.

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