Re: [PATCH v2 2/3] KVM: TDX: Set SIGNIFCANT_INDEX flag as appropriate when filling CPUID entries

[Sean Christopherson]

On Tue, Oct 27, 2026, Xiaoyao Li wrote:
> On 10/23/2026 5:01 AM, Sean Christopherson wrote:
> > diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> > index 271c2f94b8dd..37d088f6f7a8 100644
> > --- a/arch/x86/kvm/vmx/tdx.c
> > +++ b/arch/x86/kvm/vmx/tdx.c
> > @@ -2,6 +2,7 @@
> >  #include <linux/cleanup.h>
> >  #include <linux/cpu.h>
> >  #include <asm/cpufeature.h>
> > +#include <asm/cpuid/api.h>
> >  #include <asm/fpu/xcr.h>
> >  #include <linux/misc_cgroup.h>
> >  #include <linux/mmu_context.h>
> > @@ -163,12 +164,23 @@ static void td_init_cpuid_entry2(struct kvm_cpuid_entry2 *entry, unsigned char i
> >  
> >  	entry->function = (u32)td_conf->cpuid_config_leaves[idx];
> >  	entry->index = td_conf->cpuid_config_leaves[idx] >> 32;
> > +	entry->flags = 0;
> >  	entry->eax = (u32)td_conf->cpuid_config_values[idx][0];
> >  	entry->ebx = td_conf->cpuid_config_values[idx][0] >> 32;
> >  	entry->ecx = (u32)td_conf->cpuid_config_values[idx][1];
> >  	entry->edx = td_conf->cpuid_config_values[idx][1] >> 32;
> >  
> > -	if (entry->index == KVM_TDX_CPUID_NO_SUBLEAF)
> > +	WARN_ON_ONCE(cpuid_function_is_indexed(entry->function) !=
> > +		     (entry->index != KVM_TDX_CPUID_NO_SUBLEAF));
> 
> I think the WARN_ON_ONCE() is very likely to be triggered in the future when
> people run old kernel with new TDX modules.

Then upstream patches to make that not happen.  Intel insists on treating the TDX
Module like hardware.  So, treat new TDX Modules like new hardware.  Upstream
support for the new "hardware" before it ships, cc'd to stable@xxxxxxxxxxxxxxx
if necessary.

Because IMO, the WARN is doing exactly what it's intended to do: alerting the
admin that they are running a potentially dangerous setup.

> > +	/*
> > +	 * Explicitly zero the index when it's not significant, even though it
> > +	 * _should_ be meaningless.  Userspace relies on the index being zero
> > +	 * (the ABI was inherited from KVM_GET_SUPPORTED_CPUID).
> > +	 */
> > +	if (cpuid_function_is_indexed(entry->function))
> > +		entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> > +	else
> >  		entry->index = 0;
> >  
> >  	/*
> 
>