Re: [PATCH] KVM: TDX: Set SIGNIFCANT_INDEX flag for supported CPUIDs

[Binbin Wu]

On 2/25/2026 5:29 AM, Changyuan Lyu wrote:

[...]

> -----------------------
> 
>>From 18b967b718911c09872c3717d8ab083fa59c4a70 Mon Sep 17 00:00:00 2001
> From: Changyuan Lyu <changyuanl@xxxxxxxxxx>
> Date: Fri, 20 Feb 2026 09:55:28 -0800
> Subject: [PATCH] KVM: TDX: Set SIGNIFCANT_INDEX flag for supported CPUIDs
> 
> Set the KVM_CPUID_FLAG_SIGNIFCANT_INDEX flag in the kvm_cpuid_entry2
> structures returned by KVM_TDX_CAPABILITIES if the CPUID is indexed.
> This ensures consistency with the CPUID entries returned by
> KVM_GET_SUPPORTED_CPUID.
> 
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Changyuan Lyu <changyuanl@xxxxxxxxxx>


Hi Sean,

When I was preparing the v5 of the TDX CPUID patch series, the internal sashiko
reported a issue:
  > @@ -2359,6 +2359,15 @@ static int setup_tdparams_cpuids(struct kvm_cpuid2 *cpuid,
  >  		if (!entry)
  >  			continue;
  >  
  > +		/*
  > +		 * Reject entries whose index doesn't match the expected one.
  > +		 * This catches userspace passing a CPUID entry with the
  > +		 * KVM_CPUID_FLAG_SIGNIFCANT_INDEX flag cleared when the index
  > +		 * is significant.
  > +		 */
  > +		if (entry->index != tmp.index)
  > +			return -EINVAL;
  
  [Severity: Medium]
  Does this unconditional check inadvertently break userspace VMMs that provide
  valid CPUID entries but leave the index field uninitialized for
  non-significant leaves?
  
  Since kvm_find_cpuid_entry2() ignores the index field when
  KVM_CPUID_FLAG_SIGNIFCANT_INDEX is cleared, VMMs are not architecturally
  required to zero the index for non-significant leaves. This check forces
  the index to be exactly 0 (since tmp.index is normalized to 0) even for
  leaves that do not have architecturally significant subleaves.


I think the issue is valid, so it needs to know whether the entry
is a subleaf or not. I think it's more reliable to do it via the information
reported by the TDX module. i.e. my patch set will rely on this patch to set
KVM_CPUID_FLAG_SIGNIFCANT_INDEX for entries that are indexed.

Will you apply this patch?
Or I can include this patch in my patch set. Also, 0x23 is still missing in
cpuid_function_is_indexed(), I can add a patch to add 0x23 as well.


> ---
>  arch/x86/kvm/vmx/tdx.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 2d7a4d52ccfb4..1c039eab2f3d8 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -167,6 +167,7 @@ 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];
> @@ -174,6 +175,9 @@ static void td_init_cpuid_entry2(struct kvm_cpuid_entry2 *entry, unsigned char i
> 
>  	if (entry->index == KVM_TDX_CPUID_NO_SUBLEAF)
>  		entry->index = 0;
> +	else
> +		entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> +
> 
>  	/*
>  	 * The TDX module doesn't allow configuring the guest phys addr bits
> --
> 2.53.0.414.gf7e9f6c205-goog