linux-kernel - Re: [PATCH] KVM: TDX: Allow userspace to return errors to guest for MAPGPA

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Message-ID: <aWgPFQOQRr3xcMjh@google.com>
Date: Wed, 14 Jan 2026 13:48:05 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Xiaoyao Li <xiaoyao.li@...el.com>
Cc: Sagi Shahar <sagis@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Kiryl Shutsemau <kas@...nel.org>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Thomas Gleixner <tglx@...nel.org>, 
	Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev, 
	Vishal Annapurve <vannapurve@...gle.com>, Michael Roth <michael.roth@....com>
Subject: Re: [PATCH] KVM: TDX: Allow userspace to return errors to guest for MAPGPA

+Mike

On Wed, Jan 14, 2026, Sean Christopherson wrote:
> On Wed, Jan 14, 2026, Xiaoyao Li wrote:
> > On 1/14/2026 8:30 AM, Sagi Shahar wrote:
> > So it needs to be
> > 
> > 	if (vcpu->run->hypercall.ret == -EBUSY)
> > 		tdvmcall_set_return_code(vcpu, TDVMCALL_STATUS_RETRY);
> > 	else
> > 		tdvmcall_set_return_code(vcpu, TDVMCALL_STATUS_INVALID_OPERAND);
> 
> No, because assuming everything except -EBUSY translates to
> TDVMCALL_STATUS_INVALID_OPERAND paints KVM back into the same corner its already
> in.  What I care most about is eliminating KVM's assumption that a non-zero
> hypercall.ret means TDVMCALL_STATUS_INVALID_OPERAND.
> 
> For the new ABI, I see two options:
> 
>  1. Translate -errno as done in this patch.
>  2. Propagate hypercall.ret directly to the TDVMCALL return code, i.e. let
>     userspace set any return code it wants.
> 
> #1 has the downside of needing KVM changes and new uAPI every time a new return
> code is supported.
> 
> #2 has the downside of preventing KVM from establishing its own ABI around the
> return code, and making the return code vendor specific.  E.g. if KVM ever wanted
> to do something in response to -EBUSY beyond propagating the error to the guest,
> then we can't reasonably do that with #2.
> 
> Whatever we do, I want to change snp_complete_psc_msr() and snp_complete_one_psc()
> in the same patch, so that whatever ABI we establish is common to TDX and SNP.
> 
> See also https://lore.kernel.org/all/Zn8YM-s0TRUk-6T-@google.com.

Aha!  Finally.  I *knew* we had discussed this more recently.  The SNP series to
add KVM_EXIT_SNP_REQ_CERTS uses a similar pattern.  Note its intentional use of
positive values, because that's what userspace sees in errno.  This code should
do the same.  Oh, and we need to choose between EAGAIN and EBUSY...

	switch (READ_ONCE(vcpu->run->snp_req_certs.ret)) {
	case 0:
		return snp_handle_guest_req(svm, control->exit_info_1,
					    control->exit_info_2);
	case ENOSPC:
		vcpu->arch.regs[VCPU_REGS_RBX] = vcpu->run->snp_req_certs.npages;
		return snp_req_certs_err(svm, SNP_GUEST_VMM_ERR_INVALID_LEN);
	case EAGAIN:
		return snp_req_certs_err(svm, SNP_GUEST_VMM_ERR_BUSY);
	case EIO:
		return snp_req_certs_err(svm, SNP_GUEST_VMM_ERR_GENERIC);
	default:
		break;
	}


https://lore.kernel.org/all/20260109231732.1160759-2-michael.roth@amd.com