linux-kernel - Re: [RFC PATCH 3/4] KVM: TDX: Exit to userspace for GetTdVmCallInfo

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Message-ID: <9421ffccdc40fb5a75921e758626354996abb8a9.camel@intel.com>
Date: Tue, 10 Jun 2025 16:50:47 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Li, Xiaoyao" <xiaoyao.li@...el.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>, "binbin.wu@...ux.intel.com"
	<binbin.wu@...ux.intel.com>
CC: "mikko.ylinen@...ux.intel.com" <mikko.ylinen@...ux.intel.com>, "Huang,
 Kai" <kai.huang@...el.com>, "Yao, Jiewen" <jiewen.yao@...el.com>, "Chatre,
 Reinette" <reinette.chatre@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"Lindgren, Tony" <tony.lindgren@...el.com>, "Shutemov, Kirill"
	<kirill.shutemov@...el.com>, "Hunter, Adrian" <adrian.hunter@...el.com>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>
Subject: Re: [RFC PATCH 3/4] KVM: TDX: Exit to userspace for GetTdVmCallInfo

On Tue, 2025-06-10 at 17:16 +0800, Xiaoyao Li wrote:
> >     A new KVM exit reason KVM_EXIT_TDX_GET_TDVMCALL_INFO and its structure
> >     are added. Userspace is required to handle the exit reason as the
> > initial
> >     support for TDX.
> 
> It doesn't look like a good and correct design.
> 
> Consider the case that userspace supports SetupEventNotifyInterrupt and 
> returns bit 1 of leaf_output[0] as 1 to KVM, and KVM returns it to TD 
> guest for TDVMCALL_GET_TD_VM_CALL_INFO. So TD guest treats it as 
> SetupEventNotifyInterrupt is support. But when TD guest issues this 
> TDVMCALL, KVM doesn't support the exit to userspace for this specific 
> leaf and userspace doesn't have chance to handle it.

Why do we need an opt-in interface instead of a way to expose which exit's are
supported by KVM? I would think the need for a TDVMCALL opt-in interface would
only come up if there was a bad guest that was making TDVMCALLs that it did not
see in GetTdVmCallInfo. So that we would actually require an opt-in is not
guaranteed. 

Another consideration could be how to handle GetQuote for an eventual TDVMCALL
opt-in interface, should it be needed. The problem would be GetQuote would be
opted in by default and make the interface weird. But we may not want to have a
TDVMCall specific opt-in interface. There could be other TDX behaviors that we
need to opt-in around. In which case the opt-in interface could be more generic,
and by implementing the TDVMCall opt-in interface ahead of time we would end up
with two opt-in interfaces instead of one.

So how about just adding a field to struct kvm_tdx_capabilities to describe the
KVM TDVMcalls? Or some other place? But don't invent an opt-in interface
until/if we need it.