KVM: x86: Disable support for Shadow Stacks if TDP is disabled

Make TDP a hard requirement for Shadow Stacks, as there are no plans to
add Shadow Stack support to the Shadow MMU. E.g. KVM hasn't been taught
to understand the magic Writable=0,Dirty=1 combination that is required
for Shadow Stack accesses, and so enabling Shadow Stacks when using
shadow paging will put the guest into an infinite #PF loop (KVM thinks the
shadow page tables have a valid mapping, hardware says otherwise).

Reviewed-by: Binbin Wu <[e-mail]>
Reviewed-by: Xiaoyao Li <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

arch/x86/kvm/cpuid.c

···
 	if (!tdp_enabled || !boot_cpu_has(X86_FEATURE_OSPKE))
 		kvm_cpu_cap_clear(X86_FEATURE_PKU);
 
+	/*
+	 * Shadow Stacks aren't implemented in the Shadow MMU.  Shadow Stack
+	 * accesses require "magic" Writable=0,Dirty=1 protection, which KVM
+	 * doesn't know how to emulate or map.
+	 */
+	if (!tdp_enabled)
+		kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
+
 	kvm_cpu_cap_init(CPUID_7_EDX,
 		F(AVX512_4VNNIW),
 		F(AVX512_4FMAPS),