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

[Isaku Yamahata]

On Tue, Feb 28, 2023 at 09:49:10PM +0000,
"Huang, Kai" <kai.huang@xxxxxxxxx> wrote:

> 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@xxxxxxxxx> 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.

This is moot. The current check does only check maxphys address bit size and
specified xfeatures are supported by host.  It's bare minimum for kvm to work.
It doesn't try to check consistency.


> 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.

The x86 common code for KVM_CREATE_VCPU, kvm_arch_vcpu_create(), calls vcpu_create,
creates lapic, and calls vcpu_reset(). 

Setting ACPI BASE MSR with X2APIC enabled, checks if cpuid x2apic bit is set.
Please notice guest_cpuid_has(vcpu, X86_FEATURE_X2APIC) in kvm_set_apic_base().
To work around it, one way is set cpuid artificially in create method as this
patch does.  Other way would be to introduce another version of
kvm_set_apic_base() that doesn't check cpuid dedicated for this purpose.
The third option is to make it user space responsibility to set initial reset
value of APIC BASE MSR.

Which option do you prefer?

Thanks,

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

-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxx>