KVM: selftests: Force load all supported XSAVE state in state test

Extend x86's state to forcefully load *all* host-supported xfeatures by
modifying xstate_bv in the saved state.  Stuffing xstate_bv ensures that
the selftest is verifying KVM's full ABI regardless of whether or not the
guest code is successful in getting various xfeatures out of their INIT
state, e.g. see the disaster that is/was MPX.

Link: https://lore.kernel.org/r/20230928001956.924301-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/include/x86_64/processor.h

@@ -922,6 +922,15 @@ static inline bool kvm_pmu_has(struct kvm_x86_pmu_feature feature)
 	       !kvm_cpu_has(feature.anti_feature);
 }
 
+static __always_inline uint64_t kvm_cpu_supported_xcr0(void)
+{
+	if (!kvm_cpu_has_p(X86_PROPERTY_SUPPORTED_XCR0_LO))
+		return 0;
+
+	return kvm_cpu_property(X86_PROPERTY_SUPPORTED_XCR0_LO) |
+	       ((uint64_t)kvm_cpu_property(X86_PROPERTY_SUPPORTED_XCR0_HI) << 32);
+}
+
 static inline size_t kvm_cpuid2_size(int nr_entries)
 {
 	return sizeof(struct kvm_cpuid2) +

tools/testing/selftests/kvm/x86_64/state_test.c

@@ -230,6 +230,7 @@ static void __attribute__((__flatten__)) guest_code(void *arg)
 
 int main(int argc, char *argv[])
 {
+	uint64_t *xstate_bv, saved_xstate_bv;
 	vm_vaddr_t nested_gva = 0;
 	struct kvm_cpuid2 empty_cpuid = {};
 	struct kvm_regs regs1, regs2;
@@ -294,12 +295,25 @@ int main(int argc, char *argv[])
 		 * allow KVM_SET_XSAVE regardless of guest CPUID.  Manually
 		 * load only XSAVE state, MSRs in particular have a much more
 		 * convoluted ABI.
+		 *
+		 * Load two versions of XSAVE state: one with the actual guest
+		 * XSAVE state, and one with all supported features forced "on"
+		 * in xstate_bv, e.g. to ensure that KVM allows loading all
+		 * supported features, even if something goes awry in saving
+		 * the original snapshot.
 		 */
+		xstate_bv = (void *)&((uint8_t *)state->xsave->region)[512];
+		saved_xstate_bv = *xstate_bv;
+
 		vcpuN = __vm_vcpu_add(vm, vcpu->id + 1);
 		vcpu_xsave_set(vcpuN, state->xsave);
+		*xstate_bv = kvm_cpu_supported_xcr0();
+		vcpu_xsave_set(vcpuN, state->xsave);
 
 		vcpu_init_cpuid(vcpuN, &empty_cpuid);
 		vcpu_xsave_set(vcpuN, state->xsave);
+		*xstate_bv = saved_xstate_bv;
+		vcpu_xsave_set(vcpuN, state->xsave);
 
 		kvm_x86_state_cleanup(state);