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Peter Anvin" , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-coco@lists.linux.dev, Binbin Wu , Changyuan Lyu , Sashiko References: <20261022210134.612848-1-seanjc@google.com> <20261022210134.612848-3-seanjc@google.com> Content-Language: en-US From: Xiaoyao Li In-Reply-To: <20261022210134.612848-3-seanjc@google.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/23/2026 5:01 AM, Sean Christopherson wrote: > From: Changyuan Lyu > > Set the KVM_CPUID_FLAG_SIGNIFCANT_INDEX flag in the kvm_cpuid_entry2 > structures returned by KVM_TDX_CAPABILITIES if the CPUID is indexed, > otherwise KVM could confuse userspace and induce a configuration error. > This also ensures consistency with the CPUID entries returned by > KVM_GET_SUPPORTED_CPUID. > > WARN if KVM and the TDX Module don't align on which functions have > significant indices, e.g. to help ensure the kernel is updated when the > TDX Module learns about new CPUID leaves, and vice versa. > > Opportunistically add a comment to call out that zeroing the index when it > isn't significant is ABI that userspace relies on, e.g. QEMU looks for '0' > instead of checking KVM_CPUID_FLAG_SIGNIFCANT_INDEX. > > Fixes: 61bb28279623 ("KVM: TDX: Get system-wide info about TDX module on initialization") > Cc: stable@vger.kernel.org > Cc: Binbin Wu > Suggested-by: Sean Christopherson > Signed-off-by: Changyuan Lyu > [sean: keep the WARN, don't key off KVM_TDX_CPUID_NO_SUBLEAF, add comment] > Signed-off-by: Sean Christopherson > --- > arch/x86/kvm/vmx/tdx.c | 14 +++++++++++++- > 1 file changed, 13 insertions(+), 1 deletion(-) > > 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 > #include > #include > +#include > #include > #include > #include > @@ -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. > + /* > + * 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; > > /*