linux-kernel - Re: [PATCH v11 023/113] KVM: TDX: allocate/free TDX vcpu structure

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Date:   Tue, 28 Feb 2023 21:49:10 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>
CC:     "Christopherson,, Sean" <seanjc@...gle.com>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Aktas, Erdem" <erdemaktas@...gle.com>,
        "dmatlack@...gle.com" <dmatlack@...gle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [PATCH v11 023/113] KVM: TDX: allocate/free TDX vcpu structure

On Tue, 2023-02-28 at 12:18 -0800, Isaku Yamahata wrote:
> On Tue, Feb 28, 2023 at 11:52:59AM +0000,
> "Huang, Kai" <kai.huang@...el.com> wrote:
> 
> > On Tue, 2023-02-28 at 03:06 -0800, Isaku Yamahata wrote:
> > > > > +	if (!e)
> > > > > +		return -ENOMEM;
> > > > > +	*e  = (struct kvm_cpuid_entry2) {
> > > > > +		.function = 1,	/* Features for X2APIC */
> > > > > +		.index = 0,
> > > > > +		.eax = 0,
> > > > > +		.ebx = 0,
> > > > > +		.ecx = 1ULL << 21,	/* X2APIC */
> > > > > +		.edx = 0,
> > > > > +	};
> > > > > +	vcpu->arch.cpuid_entries = e;
> > > > > +	vcpu->arch.cpuid_nent = 1;
> > > > 
> > > > As mentioned above, why doing it here? Won't be this be overwritten later in
> > > > KVM_SET_CPUID2?
> > > 
> > > Yes, user space VMM can overwrite cpuid[0x1] and APIC base MSR.  But it
> > > doesn't
> > > matter because it's a bug of user space VMM. user space VMM has to keep the
> > > consistency of cpuid and MSRs.
> > > Because TDX module virtualizes cpuid[0x1].x2apic to fixed 1, KVM value doesn't
> > > matter after vcpu creation.
> > > Because KVM virtualizes APIC base as read only to guest, cpuid[0x1].x2apic
> > > doesn't matter after vcpu creation as long as user space VMM keeps KVM APIC
> > > BASE
> > > value.
> > > 
> > 
> > Contrary, can we depend on userspace VMM to set x2APIC in CPUID, but not do this
> > in KVM?  If userspace doesn't do it, we treat it as userspace's bug.
> > 
> > Plus, userspace anyway needs to set x2APIC in CPUID regardless whether you have
> > done above here, correct?
> > 
> > I don't see the point of doing above in KVM because you are neither enforcing
> > anything in KVM, nor you are reducing effort of userspace.
> 
> Good idea. I can drop cpuid part from tdx_vcpu_create() and apic base part from
> tdx_vcpu_reset(). It needs to modify tdx_has_emulated_msr() to allow user space
> VMM to update APIC BASE MSR.

My personal preference would be:

1) In KVM_SET_CPUID2, we do sanity check of CPUIDs provided by userspace, and
return error if not met (i.e X2APIC isn't advertised).  We already have cases
that KVM_SET_CPUID2 can fail, so extending to do TDX-specific check seems
reasonable to me too.
2) For APIC_BASE, you can just initialize the MSR in tdx_vcpu_reset() and ignore
any update (+pr_warn()?) to MSR_IA32_APIC_BASE.


But Sean may have different opinion especially for the CPUID part.