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

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Message-ID: <1be47d7f9d4b812c110572d8b413ecdbb537ceb7.camel@intel.com>
Date: Wed, 16 Oct 2024 17:42:05 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "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>, "Li, Xiaoyao" <xiaoyao.li@...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 Wed, 2024-10-09 at 18:01 +0300, Adrian Hunter wrote:
> tdh_vp_init_apicid() passes x2APIC ID to TDH.VP.INIT which
> is one of the steps for the TDX Module to support topology
> information for the guest i.e. CPUID leaf 0xB and CPUID leaf 0x1F.
> 
> If the host VMM does not provide CPUID leaf 0x1F values
> (i.e. the values are 0), then the TDX Module will use native
> values for both CPUID leaf 0x1F and CPUID leaf 0xB.
> 
> To get 0x1F/0xB the guest must also opt-in by setting
> TDCS.TD_CTLS.ENUM_TOPOLOGY to 1.  AFAICT currently Linux
> does not do that.
> 
> In the tdh_vp_init() case, topology information will not be
> supported.
> 
> If topology information is not supported CPUID leaf 0xB and
> CPUID leaf 0x1F will #VE, and a Linux guest will return zeros.
> 
> So, yes, it seems like tdh_vp_init_apicid() should only
> be called if there is non-zero CPUID leaf 0x1F values provided
> by host VMM. e.g. add a helper function
> 
> bool tdx_td_enum_topology(struct kvm_cpuid2 *cpuid)
> {
> 	const struct tdx_sys_info_features *modinfo = &tdx_sysinfo->features;
> 	const struct kvm_cpuid_entry2 *entry;
> 
> 	if (!(modinfo->tdx_features0 & MD_FIELD_ID_FEATURES0_TOPOLOGY_ENUM))
> 		return false;
> 
> 	entry = kvm_find_cpuid_entry2(cpuid->entries, cpuid->nent, 0x1f, 0);
> 	if (!entry)
> 		return false;
> 
> 	return entry->eax || entry->ebx || entry->ecx || entry->edx;
> }

KVM usually leaves it up to userspace to not create nonsensical VMs. So I think
we can skip the check in KVM.

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.