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

[Sean Christopherson <seanjc@google.com>]

On Fri, Oct 23, 2026, Binbin Wu 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));
> > +
> > +	/*
> > +	 * 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))
> 
> If the information about whether the entry is indexed mismatched, i.e. the
> warning above fired, should we use the information from the TDX module as the
> standard?

IMO, no.  All of this information is available at module load.  If we want to
harden KVM (and we probably should), then we should apply these sanity checks
(or equivalents) during tdx_hardware_setup() and disable TDX if any of them fail.

Anything else is a hope and a prayer.