KVM: x86: Treat MONTIOR/MWAIT as a "partially emulated" feature

Enumerate MWAIT in cpuid_func_emulated(), but only if the caller wants to
include "partially emulated" features, i.e. features that KVM kinda sorta
emulates, but with major caveats.  This will allow initializing the guest
cpu_caps based on the set of features that KVM virtualizes and/or emulates,
without needing to handle things like MONITOR/MWAIT as one-off exceptions.

Adding one-off handling for individual features is quite painful,
especially when considering future hardening.  It's very doable to verify,
at compile time, that every CPUID-based feature that KVM queries when
emulating guest behavior is actually known to KVM, e.g. to prevent KVM
bugs where KVM emulates some feature but fails to advertise support to
userspace.  In other words, any features that are special cased, i.e. not
handled generically in the CPUID framework, would also need to be special
cased for any hardening efforts that build on said framework.

Link: https://lore.kernel.org/r/20241128013424.4096668-44-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/cpuid.c

@@ -1213,7 +1213,8 @@ static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array,
 	return entry;
 }
 
-static int cpuid_func_emulated(struct kvm_cpuid_entry2 *entry, u32 func)
+static int cpuid_func_emulated(struct kvm_cpuid_entry2 *entry, u32 func,
+			       bool include_partially_emulated)
 {
 	memset(entry, 0, sizeof(*entry));
 
@@ -1227,6 +1228,16 @@ static int cpuid_func_emulated(struct kvm_cpuid_entry2 *entry, u32 func)
 		return 1;
 	case 1:
 		entry->ecx = feature_bit(MOVBE);
+		/*
+		 * KVM allows userspace to enumerate MONITOR+MWAIT support to
+		 * the guest, but the MWAIT feature flag is never advertised
+		 * to userspace because MONITOR+MWAIT aren't virtualized by
+		 * hardware, can't be faithfully emulated in software (KVM
+		 * emulates them as NOPs), and allowing the guest to execute
+		 * them natively requires enabling a per-VM capability.
+		 */
+		if (include_partially_emulated)
+			entry->ecx |= feature_bit(MWAIT);
 		return 1;
 	case 7:
 		entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
@@ -1244,7 +1255,7 @@ static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
 	if (array->nent >= array->maxnent)
 		return -E2BIG;
 
-	array->nent += cpuid_func_emulated(&array->entries[array->nent], func);
+	array->nent += cpuid_func_emulated(&array->entries[array->nent], func, false);
 	return 0;
 }