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

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Message-ID: <aEh0oGeh96n9OvCT@google.com>
Date: Tue, 10 Jun 2025 12:58:08 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Rick P Edgecombe <rick.p.edgecombe@...el.com>
Cc: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.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 Tue, Jun 10, 2025, Rick P Edgecombe wrote:
> On Tue, 2025-06-10 at 10:14 +0800, Binbin Wu wrote:
> > Check userspace has enabled KVM exit on KVM_HC_MAP_GPA_RANGE during
> > KVM_TDX_FINALIZE_VM.
> > 
> > TDVMCALL_MAP_GPA is one of the GHCI base TDVMCALLs, so it must be
> > implemented by VMM to support TDX guests. KVM converts TDVMCALL_MAP_GPA
> > to KVM_HC_MAP_GPA_RANGE, which requires userspace to enable
> > KVM_CAP_EXIT_HYPERCALL with KVM_HC_MAP_GPA_RANGE bit set. Check it when
> > userspace requests KVM_TDX_FINALIZE_VM, so that there is no need to check
> > it during TDX guests running.
> > 
> > Signed-off-by: Binbin Wu <binbin.wu@...ux.intel.com>
> 
> Do we need this change? It seems reasonable, but I don't think we need KVM to
> ensure that userspace creates a TD that meets the GHCI spec.

+1.  We do need to be careful about unintentionally creating ABI, but generally
speaking KVM shouldn't police userspace.

> So I'm not sure about the justification.
> 
> 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.