Merge tag 'kvm-x86-cet-6.18' of https://github.com/kvm-x86/linux into HEAD

KVM x86 CET virtualization support for 6.18

Add support for virtualizing Control-flow Enforcement Technology (CET) on
Intel (Shadow Stacks and Indirect Branch Tracking) and AMD (Shadow Stacks).

CET is comprised of two distinct features, Shadow Stacks (SHSTK) and Indirect
Branch Tracking (IBT), that can be utilized by software to help provide
Control-flow integrity (CFI).  SHSTK defends against backward-edge attacks
(a.k.a. Return-oriented programming (ROP)), while IBT defends against
forward-edge attacks (a.k.a. similarly CALL/JMP-oriented programming (COP/JOP)).

Attackers commonly use ROP and COP/JOP methodologies to redirect the control-
flow to unauthorized targets in order to execute small snippets of code,
a.k.a. gadgets, of the attackers choice.  By chaining together several gadgets,
an attacker can perform arbitrary operations and circumvent the system's
defenses.

SHSTK defends against backward-edge attacks, which execute gadgets by modifying
the stack to branch to the attacker's target via RET, by providing a second
stack that is used exclusively to track control transfer operations.  The
shadow stack is separate from the data/normal stack, and can be enabled
independently in user and kernel mode.

When SHSTK is is enabled, CALL instructions push the return address on both the
data and shadow stack. RET then pops the return address from both stacks and
compares the addresses.  If the return addresses from the two stacks do not
match, the CPU generates a Control Protection (#CP) exception.

IBT defends against backward-edge attacks, which branch to gadgets by executing
indirect CALL and JMP instructions with attacker controlled register or memory
state, by requiring the target of indirect branches to start with a special
marker instruction, ENDBRANCH.  If an indirect branch is executed and the next
instruction is not an ENDBRANCH, the CPU generates a #CP.  Note, ENDBRANCH
behaves as a NOP if IBT is disabled or unsupported.

From a virtualization perspective, CET presents several problems.  While SHSTK
and IBT have two layers of enabling, a global control in the form of a CR4 bit,
and a per-feature control in user and kernel (supervisor) MSRs (U_CET and S_CET
respectively), the {S,U}_CET MSRs can be context switched via XSAVES/XRSTORS.
Practically speaking, intercepting and emulating XSAVES/XRSTORS is not a viable
option due to complexity, and outright disallowing use of XSTATE to context
switch SHSTK/IBT state would render the features unusable to most guests.

To limit the overall complexity without sacrificing performance or usability,
simply ignore the potential virtualization hole, but ensure that all paths in
KVM treat SHSTK/IBT as usable by the guest if the feature is supported in
hardware, and the guest has access to at least one of SHSTK or IBT.  I.e. allow
userspace to advertise one of SHSTK or IBT if both are supported in hardware,
even though doing so would allow a misbehaving guest to use the unadvertised
feature.

Fully emulating SHSTK and IBT would also require significant complexity, e.g.
to track and update branch state for IBT, and shadow stack state for SHSTK.
Given that emulating large swaths of the guest code stream isn't necessary on
modern CPUs, punt on emulating instructions that meaningful impact or consume
SHSTK or IBT.  However, instead of doing nothing, explicitly reject emulation
of such instructions so that KVM's emulator can't be abused to circumvent CET.
Disable support for SHSTK and IBT if KVM is configured such that emulation of
arbitrary guest instructions may be required, specifically if Unrestricted
Guest (Intel only) is disabled, or if KVM will emulate a guest.MAXPHYADDR that
is smaller than host.MAXPHYADDR.

Lastly disable SHSTK support if shadow paging is enabled, as the protections
for the shadow stack are novel (shadow stacks require Writable=0,Dirty=1, so
that they can't be directly modified by software), i.e. would require
non-trivial support in the Shadow MMU.

Note, AMD CPUs currently only support SHSTK.  Explicitly disable IBT support
so that KVM doesn't over-advertise if AMD CPUs add IBT, and virtualizing IBT
in SVM requires KVM modifications.

3 files changed, 495 insertions, 0 deletions

diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
index 8926ff6808cf..148d427ff24b 100644
--- a/tools/testing/selftests/kvm/Makefile.kvm
+++ b/tools/testing/selftests/kvm/Makefile.kvm
@@ -87,6 +87,7 @@ TEST_GEN_PROGS_x86 += x86/kvm_clock_test
 TEST_GEN_PROGS_x86 += x86/kvm_pv_test
 TEST_GEN_PROGS_x86 += x86/kvm_buslock_test
 TEST_GEN_PROGS_x86 += x86/monitor_mwait_test
+TEST_GEN_PROGS_x86 += x86/msrs_test
 TEST_GEN_PROGS_x86 += x86/nested_emulation_test
 TEST_GEN_PROGS_x86 += x86/nested_exceptions_test
 TEST_GEN_PROGS_x86 += x86/platform_info_test
diff --git a/tools/testing/selftests/kvm/include/x86/processor.h b/tools/testing/selftests/kvm/include/x86/processor.h
index fbe875eafca5..51cd84b9ca66 100644
--- a/tools/testing/selftests/kvm/include/x86/processor.h
+++ b/tools/testing/selftests/kvm/include/x86/processor.h
@@ -1362,6 +1362,11 @@ static inline bool kvm_is_unrestricted_guest_enabled(void)
 	return get_kvm_intel_param_bool("unrestricted_guest");
 }
 
+static inline bool kvm_is_ignore_msrs(void)
+{
+	return get_kvm_param_bool("ignore_msrs");
+}
+
 uint64_t *__vm_get_page_table_entry(struct kvm_vm *vm, uint64_t vaddr,
 				    int *level);
 uint64_t *vm_get_page_table_entry(struct kvm_vm *vm, uint64_t vaddr);
diff --git a/tools/testing/selftests/kvm/x86/msrs_test.c b/tools/testing/selftests/kvm/x86/msrs_test.c
new file mode 100644
index 000000000000..40d918aedce6
--- /dev/null
+++ b/tools/testing/selftests/kvm/x86/msrs_test.c
@@ -0,0 +1,489 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <asm/msr-index.h>
+
+#include <stdint.h>
+
+#include "kvm_util.h"
+#include "processor.h"
+
+/* Use HYPERVISOR for MSRs that are emulated unconditionally (as is HYPERVISOR). */
+#define X86_FEATURE_NONE X86_FEATURE_HYPERVISOR
+
+struct kvm_msr {
+	const struct kvm_x86_cpu_feature feature;
+	const struct kvm_x86_cpu_feature feature2;
+	const char *name;
+	const u64 reset_val;
+	const u64 write_val;
+	const u64 rsvd_val;
+	const u32 index;
+	const bool is_kvm_defined;
+};
+
+#define ____MSR_TEST(msr, str, val, rsvd, reset, feat, f2, is_kvm)	\
+{									\
+	.index = msr,							\
+	.name = str,							\
+	.write_val = val,						\
+	.rsvd_val = rsvd,						\
+	.reset_val = reset,						\
+	.feature = X86_FEATURE_ ##feat,					\
+	.feature2 = X86_FEATURE_ ##f2,					\
+	.is_kvm_defined = is_kvm,					\
+}
+
+#define __MSR_TEST(msr, str, val, rsvd, reset, feat)			\
+	____MSR_TEST(msr, str, val, rsvd, reset, feat, feat, false)
+
+#define MSR_TEST_NON_ZERO(msr, val, rsvd, reset, feat)			\
+	__MSR_TEST(msr, #msr, val, rsvd, reset, feat)
+
+#define MSR_TEST(msr, val, rsvd, feat)					\
+	__MSR_TEST(msr, #msr, val, rsvd, 0, feat)
+
+#define MSR_TEST2(msr, val, rsvd, feat, f2)				\
+	____MSR_TEST(msr, #msr, val, rsvd, 0, feat, f2, false)
+
+/*
+ * Note, use a page aligned value for the canonical value so that the value
+ * is compatible with MSRs that use bits 11:0 for things other than addresses.
+ */
+static const u64 canonical_val = 0x123456789000ull;
+
+/*
+ * Arbitrary value with bits set in every byte, but not all bits set.  This is
+ * also a non-canonical value, but that's coincidental (any 64-bit value with
+ * an alternating 0s/1s pattern will be non-canonical).
+ */
+static const u64 u64_val = 0xaaaa5555aaaa5555ull;
+
+#define MSR_TEST_CANONICAL(msr, feat)					\
+	__MSR_TEST(msr, #msr, canonical_val, NONCANONICAL, 0, feat)
+
+#define MSR_TEST_KVM(msr, val, rsvd, feat)				\
+	____MSR_TEST(KVM_REG_ ##msr, #msr, val, rsvd, 0, feat, feat, true)
+
+/*
+ * The main struct must be scoped to a function due to the use of structures to
+ * define features.  For the global structure, allocate enough space for the
+ * foreseeable future without getting too ridiculous, to minimize maintenance
+ * costs (bumping the array size every time an MSR is added is really annoying).
+ */
+static struct kvm_msr msrs[128];
+static int idx;
+
+static bool ignore_unsupported_msrs;
+
+static u64 fixup_rdmsr_val(u32 msr, u64 want)
+{
+	/*
+	 * AMD CPUs drop bits 63:32 on some MSRs that Intel CPUs support.  KVM
+	 * is supposed to emulate that behavior based on guest vendor model
+	 * (which is the same as the host vendor model for this test).
+	 */
+	if (!host_cpu_is_amd)
+		return want;
+
+	switch (msr) {
+	case MSR_IA32_SYSENTER_ESP:
+	case MSR_IA32_SYSENTER_EIP:
+	case MSR_TSC_AUX:
+		return want & GENMASK_ULL(31, 0);
+	default:
+		return want;
+	}
+}
+
+static void __rdmsr(u32 msr, u64 want)
+{
+	u64 val;
+	u8 vec;
+
+	vec = rdmsr_safe(msr, &val);
+	__GUEST_ASSERT(!vec, "Unexpected %s on RDMSR(0x%x)", ex_str(vec), msr);
+
+	__GUEST_ASSERT(val == want, "Wanted 0x%lx from RDMSR(0x%x), got 0x%lx",
+		       want, msr, val);
+}
+
+static void __wrmsr(u32 msr, u64 val)
+{
+	u8 vec;
+
+	vec = wrmsr_safe(msr, val);
+	__GUEST_ASSERT(!vec, "Unexpected %s on WRMSR(0x%x, 0x%lx)",
+		       ex_str(vec), msr, val);
+	__rdmsr(msr, fixup_rdmsr_val(msr, val));
+}
+
+static void guest_test_supported_msr(const struct kvm_msr *msr)
+{
+	__rdmsr(msr->index, msr->reset_val);
+	__wrmsr(msr->index, msr->write_val);
+	GUEST_SYNC(fixup_rdmsr_val(msr->index, msr->write_val));
+
+	__rdmsr(msr->index, msr->reset_val);
+}
+
+static void guest_test_unsupported_msr(const struct kvm_msr *msr)
+{
+	u64 val;
+	u8 vec;
+
+	/*
+	 * KVM's ABI with respect to ignore_msrs is a mess and largely beyond
+	 * repair, just skip the unsupported MSR tests.
+	 */
+	if (ignore_unsupported_msrs)
+		goto skip_wrmsr_gp;
+
+	/*
+	 * {S,U}_CET exist if IBT or SHSTK is supported, but with bits that are
+	 * writable only if their associated feature is supported.  Skip the
+	 * RDMSR #GP test if the secondary feature is supported, but perform
+	 * the WRMSR #GP test as the to-be-written value is tied to the primary
+	 * feature.  For all other MSRs, simply do nothing.
+	 */
+	if (this_cpu_has(msr->feature2)) {
+		if  (msr->index != MSR_IA32_U_CET &&
+		     msr->index != MSR_IA32_S_CET)
+			goto skip_wrmsr_gp;
+
+		goto skip_rdmsr_gp;
+	}
+
+	vec = rdmsr_safe(msr->index, &val);
+	__GUEST_ASSERT(vec == GP_VECTOR, "Wanted #GP on RDMSR(0x%x), got %s",
+		       msr->index, ex_str(vec));
+
+skip_rdmsr_gp:
+	vec = wrmsr_safe(msr->index, msr->write_val);
+	__GUEST_ASSERT(vec == GP_VECTOR, "Wanted #GP on WRMSR(0x%x, 0x%lx), got %s",
+		       msr->index, msr->write_val, ex_str(vec));
+
+skip_wrmsr_gp:
+	GUEST_SYNC(0);
+}
+
+void guest_test_reserved_val(const struct kvm_msr *msr)
+{
+	/* Skip reserved value checks as well, ignore_msrs is trully a mess. */
+	if (ignore_unsupported_msrs)
+		return;
+
+	/*
+	 * If the CPU will truncate the written value (e.g. SYSENTER on AMD),
+	 * expect success and a truncated value, not #GP.
+	 */
+	if (!this_cpu_has(msr->feature) ||
+	    msr->rsvd_val == fixup_rdmsr_val(msr->index, msr->rsvd_val)) {
+		u8 vec = wrmsr_safe(msr->index, msr->rsvd_val);
+
+		__GUEST_ASSERT(vec == GP_VECTOR,
+			       "Wanted #GP on WRMSR(0x%x, 0x%lx), got %s",
+			       msr->index, msr->rsvd_val, ex_str(vec));
+	} else {
+		__wrmsr(msr->index, msr->rsvd_val);
+		__wrmsr(msr->index, msr->reset_val);
+	}
+}
+
+static void guest_main(void)
+{
+	for (;;) {
+		const struct kvm_msr *msr = &msrs[READ_ONCE(idx)];
+
+		if (this_cpu_has(msr->feature))
+			guest_test_supported_msr(msr);
+		else
+			guest_test_unsupported_msr(msr);
+
+		if (msr->rsvd_val)
+			guest_test_reserved_val(msr);
+
+		GUEST_SYNC(msr->reset_val);
+	}
+}
+
+static bool has_one_reg;
+static bool use_one_reg;
+
+#define KVM_X86_MAX_NR_REGS	1
+
+static bool vcpu_has_reg(struct kvm_vcpu *vcpu, u64 reg)
+{
+	struct {
+		struct kvm_reg_list list;
+		u64 regs[KVM_X86_MAX_NR_REGS];
+	} regs = {};
+	int r, i;
+
+	/*
+	 * If KVM_GET_REG_LIST succeeds with n=0, i.e. there are no supported
+	 * regs, then the vCPU obviously doesn't support the reg.
+	 */
+	r = __vcpu_ioctl(vcpu, KVM_GET_REG_LIST, &regs.list);
+	if (!r)
+		return false;
+
+	TEST_ASSERT_EQ(errno, E2BIG);
+
+	/*
+	 * KVM x86 is expected to support enumerating a relative small number
+	 * of regs.  The majority of registers supported by KVM_{G,S}ET_ONE_REG
+	 * are enumerated via other ioctls, e.g. KVM_GET_MSR_INDEX_LIST.  For
+	 * simplicity, hardcode the maximum number of regs and manually update
+	 * the test as necessary.
+	 */
+	TEST_ASSERT(regs.list.n <= KVM_X86_MAX_NR_REGS,
+		    "KVM reports %llu regs, test expects at most %u regs, stale test?",
+		    regs.list.n, KVM_X86_MAX_NR_REGS);
+
+	vcpu_ioctl(vcpu, KVM_GET_REG_LIST, &regs.list);
+	for (i = 0; i < regs.list.n; i++) {
+		if (regs.regs[i] == reg)
+			return true;
+	}
+
+	return false;
+}
+
+static void host_test_kvm_reg(struct kvm_vcpu *vcpu)
+{
+	bool has_reg = vcpu_cpuid_has(vcpu, msrs[idx].feature);
+	u64 reset_val = msrs[idx].reset_val;
+	u64 write_val = msrs[idx].write_val;
+	u64 rsvd_val = msrs[idx].rsvd_val;
+	u32 reg = msrs[idx].index;
+	u64 val;
+	int r;
+
+	if (!use_one_reg)
+		return;
+
+	TEST_ASSERT_EQ(vcpu_has_reg(vcpu, KVM_X86_REG_KVM(reg)), has_reg);
+
+	if (!has_reg) {
+		r = __vcpu_get_reg(vcpu, KVM_X86_REG_KVM(reg), &val);
+		TEST_ASSERT(r && errno == EINVAL,
+			    "Expected failure on get_reg(0x%x)", reg);
+		rsvd_val = 0;
+		goto out;
+	}
+
+	val = vcpu_get_reg(vcpu, KVM_X86_REG_KVM(reg));
+	TEST_ASSERT(val == reset_val, "Wanted 0x%lx from get_reg(0x%x), got 0x%lx",
+		    reset_val, reg, val);
+
+	vcpu_set_reg(vcpu, KVM_X86_REG_KVM(reg), write_val);
+	val = vcpu_get_reg(vcpu, KVM_X86_REG_KVM(reg));
+	TEST_ASSERT(val == write_val, "Wanted 0x%lx from get_reg(0x%x), got 0x%lx",
+		    write_val, reg, val);
+
+out:
+	r = __vcpu_set_reg(vcpu, KVM_X86_REG_KVM(reg), rsvd_val);
+	TEST_ASSERT(r, "Expected failure on set_reg(0x%x, 0x%lx)", reg, rsvd_val);
+}
+
+static void host_test_msr(struct kvm_vcpu *vcpu, u64 guest_val)
+{
+	u64 reset_val = msrs[idx].reset_val;
+	u32 msr = msrs[idx].index;
+	u64 val;
+
+	if (!kvm_cpu_has(msrs[idx].feature))
+		return;
+
+	val = vcpu_get_msr(vcpu, msr);
+	TEST_ASSERT(val == guest_val, "Wanted 0x%lx from get_msr(0x%x), got 0x%lx",
+		    guest_val, msr, val);
+
+	if (use_one_reg)
+		vcpu_set_reg(vcpu, KVM_X86_REG_MSR(msr), reset_val);
+	else
+		vcpu_set_msr(vcpu, msr, reset_val);
+
+	val = vcpu_get_msr(vcpu, msr);
+	TEST_ASSERT(val == reset_val, "Wanted 0x%lx from get_msr(0x%x), got 0x%lx",
+		    reset_val, msr, val);
+
+	if (!has_one_reg)
+		return;
+
+	val = vcpu_get_reg(vcpu, KVM_X86_REG_MSR(msr));
+	TEST_ASSERT(val == reset_val, "Wanted 0x%lx from get_reg(0x%x), got 0x%lx",
+		    reset_val, msr, val);
+}
+
+static void do_vcpu_run(struct kvm_vcpu *vcpu)
+{
+	struct ucall uc;
+
+	for (;;) {
+		vcpu_run(vcpu);
+
+		switch (get_ucall(vcpu, &uc)) {
+		case UCALL_SYNC:
+			host_test_msr(vcpu, uc.args[1]);
+			return;
+		case UCALL_PRINTF:
+			pr_info("%s", uc.buffer);
+			break;
+		case UCALL_ABORT:
+			REPORT_GUEST_ASSERT(uc);
+		case UCALL_DONE:
+			TEST_FAIL("Unexpected UCALL_DONE");
+		default:
+			TEST_FAIL("Unexpected ucall: %lu", uc.cmd);
+		}
+	}
+}
+
+static void vcpus_run(struct kvm_vcpu **vcpus, const int NR_VCPUS)
+{
+	int i;
+
+	for (i = 0; i < NR_VCPUS; i++)
+		do_vcpu_run(vcpus[i]);
+}
+
+#define MISC_ENABLES_RESET_VAL (MSR_IA32_MISC_ENABLE_PEBS_UNAVAIL | MSR_IA32_MISC_ENABLE_BTS_UNAVAIL)
+
+static void test_msrs(void)
+{
+	const struct kvm_msr __msrs[] = {
+		MSR_TEST_NON_ZERO(MSR_IA32_MISC_ENABLE,
+				  MISC_ENABLES_RESET_VAL | MSR_IA32_MISC_ENABLE_FAST_STRING,
+				  MSR_IA32_MISC_ENABLE_FAST_STRING, MISC_ENABLES_RESET_VAL, NONE),
+		MSR_TEST_NON_ZERO(MSR_IA32_CR_PAT, 0x07070707, 0, 0x7040600070406, NONE),
+
+		/*
+		 * TSC_AUX is supported if RDTSCP *or* RDPID is supported.  Add
+		 * entries for each features so that TSC_AUX doesn't exists for
+		 * the "unsupported" vCPU, and obviously to test both cases.
+		 */
+		MSR_TEST2(MSR_TSC_AUX, 0x12345678, u64_val, RDTSCP, RDPID),
+		MSR_TEST2(MSR_TSC_AUX, 0x12345678, u64_val, RDPID, RDTSCP),
+
+		MSR_TEST(MSR_IA32_SYSENTER_CS, 0x1234, 0, NONE),
+		/*
+		 * SYSENTER_{ESP,EIP} are technically non-canonical on Intel,
+		 * but KVM doesn't emulate that behavior on emulated writes,
+		 * i.e. this test will observe different behavior if the MSR
+		 * writes are handed by hardware vs. KVM.  KVM's behavior is
+		 * intended (though far from ideal), so don't bother testing
+		 * non-canonical values.
+		 */
+		MSR_TEST(MSR_IA32_SYSENTER_ESP, canonical_val, 0, NONE),
+		MSR_TEST(MSR_IA32_SYSENTER_EIP, canonical_val, 0, NONE),
+
+		MSR_TEST_CANONICAL(MSR_FS_BASE, LM),
+		MSR_TEST_CANONICAL(MSR_GS_BASE, LM),
+		MSR_TEST_CANONICAL(MSR_KERNEL_GS_BASE, LM),
+		MSR_TEST_CANONICAL(MSR_LSTAR, LM),
+		MSR_TEST_CANONICAL(MSR_CSTAR, LM),
+		MSR_TEST(MSR_SYSCALL_MASK, 0xffffffff, 0, LM),
+
+		MSR_TEST2(MSR_IA32_S_CET, CET_SHSTK_EN, CET_RESERVED, SHSTK, IBT),
+		MSR_TEST2(MSR_IA32_S_CET, CET_ENDBR_EN, CET_RESERVED, IBT, SHSTK),
+		MSR_TEST2(MSR_IA32_U_CET, CET_SHSTK_EN, CET_RESERVED, SHSTK, IBT),
+		MSR_TEST2(MSR_IA32_U_CET, CET_ENDBR_EN, CET_RESERVED, IBT, SHSTK),
+		MSR_TEST_CANONICAL(MSR_IA32_PL0_SSP, SHSTK),
+		MSR_TEST(MSR_IA32_PL0_SSP, canonical_val, canonical_val | 1, SHSTK),
+		MSR_TEST_CANONICAL(MSR_IA32_PL1_SSP, SHSTK),
+		MSR_TEST(MSR_IA32_PL1_SSP, canonical_val, canonical_val | 1, SHSTK),
+		MSR_TEST_CANONICAL(MSR_IA32_PL2_SSP, SHSTK),
+		MSR_TEST(MSR_IA32_PL2_SSP, canonical_val, canonical_val | 1, SHSTK),
+		MSR_TEST_CANONICAL(MSR_IA32_PL3_SSP, SHSTK),
+		MSR_TEST(MSR_IA32_PL3_SSP, canonical_val, canonical_val | 1, SHSTK),
+
+		MSR_TEST_KVM(GUEST_SSP, canonical_val, NONCANONICAL, SHSTK),
+	};
+
+	const struct kvm_x86_cpu_feature feat_none = X86_FEATURE_NONE;
+	const struct kvm_x86_cpu_feature feat_lm = X86_FEATURE_LM;
+
+	/*
+	 * Create three vCPUs, but run them on the same task, to validate KVM's
+	 * context switching of MSR state.  Don't pin the task to a pCPU to
+	 * also validate KVM's handling of cross-pCPU migration.  Use the full
+	 * set of features for the first two vCPUs, but clear all features in
+	 * third vCPU in order to test both positive and negative paths.
+	 */
+	const int NR_VCPUS = 3;
+	struct kvm_vcpu *vcpus[NR_VCPUS];
+	struct kvm_vm *vm;
+	int i;
+
+	kvm_static_assert(sizeof(__msrs) <= sizeof(msrs));
+	kvm_static_assert(ARRAY_SIZE(__msrs) <= ARRAY_SIZE(msrs));
+	memcpy(msrs, __msrs, sizeof(__msrs));
+
+	ignore_unsupported_msrs = kvm_is_ignore_msrs();
+
+	vm = vm_create_with_vcpus(NR_VCPUS, guest_main, vcpus);
+
+	sync_global_to_guest(vm, msrs);
+	sync_global_to_guest(vm, ignore_unsupported_msrs);
+
+	/*
+	 * Clear features in the "unsupported features" vCPU.  This needs to be
+	 * done before the first vCPU run as KVM's ABI is that guest CPUID is
+	 * immutable once the vCPU has been run.
+	 */
+	for (idx = 0; idx < ARRAY_SIZE(__msrs); idx++) {
+		/*
+		 * Don't clear LM; selftests are 64-bit only, and KVM doesn't
+		 * honor LM=0 for MSRs that are supposed to exist if and only
+		 * if the vCPU is a 64-bit model.  Ditto for NONE; clearing a
+		 * fake feature flag will result in false failures.
+		 */
+		if (memcmp(&msrs[idx].feature, &feat_lm, sizeof(feat_lm)) &&
+		    memcmp(&msrs[idx].feature, &feat_none, sizeof(feat_none)))
+			vcpu_clear_cpuid_feature(vcpus[2], msrs[idx].feature);
+	}
+
+	for (idx = 0; idx < ARRAY_SIZE(__msrs); idx++) {
+		struct kvm_msr *msr = &msrs[idx];
+
+		if (msr->is_kvm_defined) {
+			for (i = 0; i < NR_VCPUS; i++)
+				host_test_kvm_reg(vcpus[i]);
+			continue;
+		}
+
+		/*
+		 * Verify KVM_GET_SUPPORTED_CPUID and KVM_GET_MSR_INDEX_LIST
+		 * are consistent with respect to MSRs whose existence is
+		 * enumerated via CPUID.  Skip the check for FS/GS.base MSRs,
+		 * as they aren't reported in the save/restore list since their
+		 * state is managed via SREGS.
+		 */
+		TEST_ASSERT(msr->index == MSR_FS_BASE || msr->index == MSR_GS_BASE ||
+			    kvm_msr_is_in_save_restore_list(msr->index) ==
+			    (kvm_cpu_has(msr->feature) || kvm_cpu_has(msr->feature2)),
+			    "%s %s in save/restore list, but %s according to CPUID", msr->name,
+			    kvm_msr_is_in_save_restore_list(msr->index) ? "is" : "isn't",
+			    (kvm_cpu_has(msr->feature) || kvm_cpu_has(msr->feature2)) ?
+			    "supported" : "unsupported");
+
+		sync_global_to_guest(vm, idx);
+
+		vcpus_run(vcpus, NR_VCPUS);
+		vcpus_run(vcpus, NR_VCPUS);
+	}
+
+	kvm_vm_free(vm);
+}
+
+int main(void)
+{
+	has_one_reg = kvm_has_cap(KVM_CAP_ONE_REG);
+
+	test_msrs();
+
+	if (has_one_reg) {
+		use_one_reg = true;
+		test_msrs();
+	}
+}