linux-kernel - Re: [PATCH 18/25] KVM: TDX: Do TDX specific vcpu initialization

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Message-ID: <0136dbcb2712828859447bc7696974e643a76208.camel@intel.com>
Date: Fri, 18 Oct 2024 14:20:47 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Li, Xiaoyao" <xiaoyao.li@...el.com>, "Hunter, Adrian"
	<adrian.hunter@...el.com>, "yuan.yao@...ux.intel.com"
	<yuan.yao@...ux.intel.com>
CC: "seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai"
	<kai.huang@...el.com>, "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tony.lindgren@...ux.intel.com" <tony.lindgren@...ux.intel.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>
Subject: Re: [PATCH 18/25] KVM: TDX: Do TDX specific vcpu initialization

On Fri, 2024-10-18 at 10:21 +0800, Xiaoyao Li wrote:
> > KVM usually leaves it up to userspace to not create nonsensical VMs. So I
> > think
> > we can skip the check in KVM.
> 
> It's not nonsensical unless KVM announces its own requirement for TD 
> guest that userspace VMM must provide valid CPUID leaf 0x1f value for 
> topology.

How about adding it to the docs?

> 
> It's architectural valid that userspace VMM creates a TD with legacy 
> topology, i.e., topology enumerated via CPUID 0x1 and 0x4.
> 
> > In that case, do you see a need for the vanilla tdh_vp_init() SEAMCALL
> > wrapper?
> > 
> > The TDX module version we need already supports enum_topology, so the code:
> >  	if (modinfo->tdx_features0 & MD_FIELD_ID_FEATURES0_TOPOLOGY_ENUM)
> >  		err = tdh_vp_init_apicid(tdx, vcpu_rcx, vcpu->vcpu_id);
> >  	else
> >  		err = tdh_vp_init(tdx, vcpu_rcx);
> > 
> > The tdh_vp_init() branch shouldn't be hit.
> 
> We cannot know what version of TDX module user might use thus we cannot 
> assume enum_topology is always there unless we make it a hard 
> requirement in KVM that TDX fails being enabled when
> 
>    !(modinfo->tdx_features0 & MD_FIELD_ID_FEATURES0_TOPOLOGY_ENUM)

We will depend on bugs that are fixed in TDX Modules after enum topology, so it
shouldn't be required in the normal case. So I think it would be simpler to add
this tdx_features0 conditional. We can then export one less SEAMCALL and will
have less configurations flows to worry about on the KVM side.