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

[Sean Christopherson <seanjc@google.com>]

From: Changyuan Lyu <changyuanl@google.com>

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 <binbin.wu@linux.intel.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Changyuan Lyu <changyuanl@google.com>
[sean: keep the WARN, don't key off KVM_TDX_CPUID_NO_SUBLEAF, add comment]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 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 <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))
+		entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
+	else
 		entry->index = 0;
 
 	/*
-- 
2.56.0.257.gf7497c7d27-goog