linux-kernel - Re: [RFC PATCH 4/4] KVM: TDX: Check KVM exit on KVM_HC_MAP_GPA

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Message-ID: <aEmGTZbMpZhtlkIh@google.com>
Date: Wed, 11 Jun 2025 06:36:13 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
Cc: Rick P Edgecombe <rick.p.edgecombe@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"mikko.ylinen@...ux.intel.com" <mikko.ylinen@...ux.intel.com>, Kai Huang <kai.huang@...el.com>, 
	Jiewen Yao <jiewen.yao@...el.com>, Xiaoyao Li <xiaoyao.li@...el.com>, 
	Tony Lindgren <tony.lindgren@...el.com>, Adrian Hunter <adrian.hunter@...el.com>, 
	Reinette Chatre <reinette.chatre@...el.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Yan Y Zhao <yan.y.zhao@...el.com>, 
	Isaku Yamahata <isaku.yamahata@...el.com>, Kirill Shutemov <kirill.shutemov@...el.com>
Subject: Re: [RFC PATCH 4/4] KVM: TDX: Check KVM exit on KVM_HC_MAP_GPA_RANGE
 when TD finalize

On Wed, Jun 11, 2025, Binbin Wu wrote:
> On 6/11/2025 3:58 AM, Sean Christopherson wrote:
> > On Tue, Jun 10, 2025, Rick P Edgecombe wrote:
> > > It seems like the reasoning could be just to shrink the possible configurations
> > > KVM has to think about, and that we only have the option to do this now before
> > > the ABI becomes harder to change.
> > > 
> > > Did you need any QEMU changes as a result of this patch?
> > > 
> > > Wait, actually I think the patch is wrong, because KVM_CAP_EXIT_HYPERCALL could
> > > be called again after KVM_TDX_FINALIZE_VM. In which case userspace could get an
> > > exit unexpectedly. So should we drop this patch?
> > Yes, drop it.
> > 
> So, when the TDX guest calls MapGPA and KVM finds userspace doesn't opt-in
> KVM_HC_MAP_GPA_RANGE, just return error to userspace?

Why can't KVM just do what it already does, and return an error to the guest?

	if (!user_exit_on_hypercall(vcpu->kvm, KVM_HC_MAP_GPA_RANGE)) {
		ret = TDVMCALL_STATUS_INVALID_OPERAND;
		goto error;
	}